Optical disc apparatus

ABSTRACT

An optical disc apparatus capable of mounting an optical disc includes a light source for emitting light: an objective lens for collecting the light emitted by the light source on the optical disc; a first light distribution section integrally movable with the objective lens, the first light distribution section including a first area and a second area, the first light distribution section outputting the light reflected by the optical disc and transmitted through the first area or the second area as transmission light, outputting the light reflected by the optical disc and diffracted by the first area as first diffraction light, and outputting the light reflected by the optical disc and diffracted by the second area as second diffraction light; a transmission light detection section for detecting the transmission light and outputting a TE 1  signal indicating an offset of the detected transmission light; a first diffraction light detection section for detecting the first diffraction light and the second diffraction light, and outputting a TE 2  signal indicating a difference between a light amount of the detected first diffraction light and a light amount of the detected second diffraction light; and a control device for generating a tracking error signal for the optical disc based on the TE 1  signal and the TE 2  signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical disc apparatus, andin particular to an optical disc apparatus for finding an accuratetracking error signal for an optical disc.

[0003] 2. Description of the Related Art

[0004] An optical disc is known as an information recording medium forstoring a large amount of data. An optical disc can store information ontracks thereof, and also allow information recorded thereon to bereproduced. An optical disc apparatus is capable of mounting an opticalis other on and is used for recording information on the optical discand/or reproducing information stored on the optical disc. In order toallow the optical disc apparatus to record information to or reproduceinformation from an appropriate track accurately, a laser beam needs toaccurately follow the tracks on the optical disc. The operation of thelaser beam to follow the tracks on the optical disc is referred to as“tracking”. A tracking error signal shows whether the laser beam isaccurately following the tracks on the optical disc.

[0005] Hereinafter, a conventional optical disc apparatus and a trackingerror signal provided by the conventional optical disc apparatus will bedescribed.

[0006]FIG. 10A shows a conventional optical disc apparatus 1000. Laserlight emitted by a laser light source 1010 is converged on an opticaldisc 1070 through an optical system 1015. The light reflected by theoptical disc 1070 is detected by a photodetector 1050. Based on a resultdetected by the photodetector 1050, a control device 1085 controls anelement or elements among the light source 1010, the optical system1015, and the optical disc 1070 as necessary. The optical system 1015includes, for example, a polarizing beam splitter 1020 having asplitting face 1025, a collimator lens 1030, a quarter-wave plate 1042,a reflecting mirror 1040, and an objective lens 1060.

[0007] A more specific operation of the optical disc apparatus 1000 willbe described.

[0008] Laser light emitted by the light source 1010 is incident on thepolarizing beam splitter 1020, transmitted through the splitting face1025 of the polarizing beam splitter 1020, and then converted intoparallel light by the collimator lens 1030. The parallel light, which islinearly polarized (P wave) is converted into circular polarization, bythe quarter-wave plate 1042, and then reflected by the reflecting mirror1040. The reflected light is converged by the objective lens 1060 on asignal face 1074 of the optical disc 1070.

[0009] The optical disc 1070 has the signal face 1074 between asubstrate 1072 and a protection film 1076. The signal face 1074 has pits(or grooves) formed in a diameter direction of the optical disc 1070(indicated by arrow X). The pits each have a depth d and a width w, andare arranged at a pitch p. The diameter direction of the optical disc1070 is perpendicular to the direction of the light incident on theoptical disc 1070 and parallel to the sheet of paper of FIG. 10A.

[0010] The light reflected by the signal face 1074, which is circularpolarization, is transmitted through the objective lens 1060, reflectedby the reflecting mirror 1040, and then converted into linearpolarization (S wave) by the quarter-wave plate 1042. The light is madeconvergent by the collimator lens 1030, reflected by the splitting face1025 of the polarizing beam splitter 1020, and then collected on thephotodetector 1050 as light 1080. Based on a signal detected by thephotodetector 1050, the control device 1085 controls an element orelements among the light source 1010, the optical system 1015, and theoptical disc 1070 as necessary.

[0011] In FIG. 10A, reference numeral 1210 represents an optical axis ofthe optical disc apparatus 1000.

[0012]FIG. 10B shows a structure of the photodetector 1050. Thephotodetector 1050 includes sub-photodetectors 1050A and 1050B. Aseparation line 1051 shows the border between the sub-photodetectors1050A and 1050B. The sub-photodetector 1050A and 1050B each provide arespective light amount. A tracking error signal 1091 s (TE1 signal) isobtained by subjecting the light amounts provided by thesub-photodetectors 1050A and 1050B to subtraction performed by asubtracter 1091. A reproduction signal 1092 is obtained by subjectingthe light amounts provided by the sub-photodetectors 1050A and 1050B toaddition performed by an adder 1092. The separation line 1051substantially equally divides a convergence spot 1081 on thephotodetector 1050. The control device 1085 controls an element orelements among the light source 1010, the optical system 1015, and theoptical disc 1070 as necessary, so as to make the level of the TE1signal zero in order to eliminate d tracking error.

[0013]FIG. 11A shows another conventional optical disc apparatus 1100.Laser light emitted by a laser light source 1110 is converged on anoptical disc 1170 through an optical system 1115. The light reflected bythe optical disc 1170 is detected by a photodetector 1190, Based on aresult detected by the photodetector 1190, a control device 1185controls an element or elements among the light source 1110, the opticalsystem 1115, and the optical disc 1170 as necessary. The optical system1115 includes, for example, a collimator lens 1130, a quarter-wave plate1142, a polarizing holographic element 1145, and an objective lens 1160.

[0014] A more specific operation of the optical disc apparatus 1100 willbe described.

[0015] Laser light emitted by the light source 1110 is converted intoparallel light by the collimator lens 1130 and incident on thepolarizing holographic element 1145.

[0016] The polarizing holographic element 1145 is integrated into a lensholder 116 s together with the objective lens 1160. The polarizingholographic element 1145 has the quarter-wave plate 1142. A surface ofthe polarizing holographic element 1145 is a polarizing holographic face1150.

[0017] The light, which is linear polarization (P wave) incident on thepolarizing holographic element 1145 is transmitted through thepolarizing holographic face 1150 and converted into circularpolarization by the quarter-wave plate 1142, collected by the objectivelens 1160, and then converged on a signal face 1174 of the optical disc1170.

[0018] The optical disc 1170 has the signal lace 1174 between asubstrate 1172 and a protection film 1176. The signal face 1174 has pits(or grooves) formed in a rotation direction of the optical disc 1170.The pits each have a depth d and a width w, and arranged at a pitch p.

[0019] The light reflected by the signal face 1174, which is circularpolarization, is transmitted through the objective lens 1160, convertedinto linear polarizatlon (S wave) by the quarter-wave plate 1142, andthen diffracted by the polarizing holographic face 1150. The diffractionlight is transmitted through the collimator lens 1130 and incident onthe photodetector 1190. Based on a signal detected by the photodetector1190; the control device 1185 controls an element or elements among thelight source 1110, the optical system 1115, and the optical disc 1170 asnecessary.

[0020]FIG. 11B shows a structure of the polarizing holographic face1150. The polarizing holographic face 1150 includes two areas 1150 a and1315 b which are separated from each other by a separation line 1152.The light reflected by the optical disc 1170 is substantially equallydivided into two by the separation line 1152.

[0021]FIG. 11C shows a structure of the photodetector 1190. Thephotodetector 1190 includes two sub-photodetectors 1190A and 1190Bseparated from each other by a separation line 1191. The lightdiffracted by the area 1150 a (FIG. 11B) of the polarizing holographicface 1150 is collected on the sub-photodetector 1190A as a spot 1181 a.The light diffracted by the area 1150 b (FIG. 11B) of the polarizingholographic face 1150 is collected on the sub-photodetector 1190B as aspot 1181 b. The sub-photodetectors 1190A and 1190B each provide arespective light amount. A tracking error signal 1101 s (TE2 signal) isobtained by subjecting the light amounts provided by thesub-photodetectors 1190A and 1190B to subtraction performed by asubtracter 1101. A reproduction signal 1102B is obtained by subjectingthe light amounts provided by the sub-photodetectors 1190A and 1190B toaddition performed by an adder 1102. The control device 1185 controls anelement or elements among the light source 1110, the optical system1115, and the optical disc 1170 as necessary, so as to make the level ofthe TE2 signal zero in order to eliminate a tracking error.

[0022] The tracking error signals (TE1 signal and TE2 signal) obtainedby the conventional optical disc apparatuses 1000 and 1100 have thefollowing problems. First, the tracking error signal obtained by theconventional optical disc apparatus 1000 (TE1 signal) will be described.

[0023] Generally in the optical disc 1000, in which the control device1085 performs tracking control, when the optical disc 1070 vibrates withrespect to the center thereof, the objective lens 1060 follows thevibration and is shifted in the diameter direction K (FIG. 10A).

[0024]FIG. 12 (parts (a) through (d)) shows light intensitydistributions of a cross-section of the optical disc 1070 when a centralaxis 1220 (part (e)) of the objective lens 1060 is shifted rightward bydistance X with respect the optical axis 1210 of the optical discapparatus 1000 (FIG. 1). The cross-section is taken along the diameterdirection of the optical disc apparatus 1070. Part (e) schematicallyshows the positional relationship between the optical axis 1210 and thecentral axis 1220 of the objective lens 1060.

[0025] In FIG. 12, part (a) shows a light intensity distribution 1231before the light emitted by the light source 1030 is transmitted throughthe objective lens 1060. The light intensity distribution 1231 exhibitsa Gaussian distribution with the optical axis 1210 as the center. Atthis point, as shown in part (e), the central axis 1220 of the objectivelens 1060 is shifted by distance X with respect to the optical axis 1210of the optical disc apparatus 1000.

[0026] Part (b) shows a light intensity distribution 1232 after thelight is transmitted through the objective lens 1060. When the objectivelens 1060 has a radius (aperture radius) of length r, the lightintensity distribution 1232 is zero at a position farther than distancer from the central axis 1220 of the objective lens 1060. In other words,the light outer aperture rims 1240 and 1250 of the objective lens 1060are shielded.

[0027] Part (c) shows a light intensity distribution 1233 after thelight is reflected by the optical disc 1070 and before being incident onthe objective lens 1060. A central axis 1215 of the light reflected bythe optical disc 1070 is shifted rightward by distance X with respect tothe central axis 1220 of the objective lens 1060. In other words, thecentral axis 1215 of the light reflected by the optical disc 1070 isshifted rightward by distance 2X with respect to the optical axis 1210of the optical disc apparatus 1000. The light intensity distribution1233 is spread in the diameter direction of the optical disc 1070 due tothe diffraction at the pits on the signal face 1074 of the optical discapparatus 1070.

[0028] Part (d) shows a light intensity distribution 1234 after thelight is transmitted through the objective lens 1060. As in part (b),the light outside the aperture rime 1240 and 1250 of the objective lens1060 is shielded.

[0029] When distance X is zero, the tracking of the optical disc 1070 isaccurately controlled by controlling the level of the tracking errorsignal (TE1 signal) obtained by the photodetector 1050 (FIG. 10B) to bezero. However, when distance X is not zero, a tracking offset isgenerated.

[0030] As described above, the tracking error signal (TE1 signal)obtained by the photodetector 1050 (FIG. 10B) shows a difference in thelight amounts detected by the sub-photodetectors 1050A and 1050B. When adistance X exists between the optical axis 1210 and the central axis1220 of the objective lens 1060, the light amount detected by thesub-photodetector 1050A corresponds to an area of a pattern ABCD formedby connecting points A, B, C and D (part (d)), and the light amountdetected by the sub-photodetector 1050B correspond to an area of apattern CDEF formed by connecting points C, D, E and P.

[0031] The tracking error signal (TE2 signal) obtained by thephotodetector 1190 of the optical disc apparatus 1100 (FIG. 11A) is alsoshifted in a similar manner when there is a distance between an opticalaxis of the optical disc apparatus 1100 and a central axis of theobjective lens 1160 for the following reason.

[0032] The tracking error signal (TE2 signal) obtained by thephotodetector 1190 (FIG. 11C) shows a difference in the light amountsdetected by the sub-photodetectors 1190A and 1190B. When a distance Xexists between the optical axis of the optical disc apparatus 1100 andthe central axis of the objective lens 1160, the light amount detectedby the sub-photodetector 1190A correspond to an area of a pattern formedby connecting points A, B, C′ and D′ (part (d)), and the light amountdetected by the sub-photodetector 1050B correspond to an area of apattern formed by connecting points C′, D′, E and F. The tracking errorsignal provided by the photodetector 1190 (TE2 signal) is not offset asmuch an the tracking error signal provided by the photodetector 1050(TE1 signal) but is still offset significantly.

[0033]FIG. 13A is a graph illustrating the degree of asymmetry of thewaveform of the tracking error signal when the laser light crosses thepits (when tracking is off). In FIG. 13A, distance X between the opticaladds 1210 of the optical disc apparatus 1000 and the central axis 1220of the objective lens 1060 is assumed to be 100 μm. The degree ofasymmetry is represented as contours. The degree of asymmetry isobtained by expression (H−L)/(H+L), where H is a level of the signaloutput (indicated by reference numeral 1300) shown in FIG. 13B above theground level GND, and L is a level of the signal output shown in FIG.13B below the ground level GND.

[0034] In FIG. 13A, the horizontal axis represents the width of the pitsw of the optical disc 1070, and the vertical axis represents the depthof the pits (d×refractive index of the substrate 1072 of the opticaldisc 1070, see FIG. 10A). The parameters for the calculation obtainedfor the results shown in FIG. 13A are as follows: the numerical aperture(NA) of the objective lens 1060=0.60; the wavelength λ of the lightsource 1010=0.66 μm; the pitch (P) of the pits of the optical disc1070=0.74 μm. At point R (where the width w of the pits is 0.30 μm andthe depth of the pits is λ/10), the degree of asymmetry of the trackingerror signal is 0.52. This corresponds to the difference between theareas of the pattern ABCD and the pattern CDEF shown in part (d) of FIG.12. As can be appreciated, in the optical disc apparatus 1000 includingthe photodetector 1050, the central axis 1220 of the objective lens 1060is shifted with respect to the optical axis 1210 of the optical discapparatus 1000 in the direction of arrow X (FIG. 1A). As a result, asignificant degree of asymmetry of the tracking error signal occurs, andtherefore control of tracking becomes unstable. While tracking controlis performed, very large off-track may be undesirably generated. Thiscauses a tracking error signal from an adjacent track to be leaked(i.e., crosstalk is increased) and deteriorates the reproductionperformance, or causes a part of a signal mark of an adjacent track tobe overwritten or erased.

[0035]FIG. 14 is a graph illustrating the degree of asymmetry of thewaveform of the tracking error signal generated when the photodetector1190 in the optical disc apparatus 1000 issused. The conditions are thesame as above. At point R (where the width w of the pits is 0.30 μm andthe depth of the pits is λ/10), the degree of asymmetry of the trackingerror signal is 0.18. This corresponds to the difference between theareas of the pattern ABC′D′ (and the pattern C′D′EF shown in part (d) ofFIG. 12. The degree of asymmetry is lower than that provided by thephotodetector 1050 but is still sufficiently large to cause the unstablecontrol of tracking, a significant control error (off-track), and otherproblems.

SUMMARY OF THE INVENTION

[0036] An optical disc apparatus capable of mounting an optical discaccording to the present invention includes a light source for emittinglight; an objective lens for collecting the light emitted by the lightsource on the optical disc; a first light distribution sectionintegrally movable with the objective lens, the first light distributionsection including a first area and a second area, the first lightdistribution section outputting the light reflected by the optical discand transmitted through the first area or the second area astransmission light, outputting the light reflected by the optical discand diffracted by the first area as first diffraction light, andoutputting the light reflected by the optical disc and a diffracted bythe second area as second diffraction light; a transmission lightdetection section for detecting the transmission light and outputting aTE1 signal indicating an offset of the detected transmission light; afirst diffraction light detection section for detecting the firstdiffraction light and the second diffraction light, and outputting a TE2signal indicating a difference between a light amount of the detectedfirst diffraction light and a light amount of the detected seconddiffraction light; and a control device for generating a tracking errorsignal for the optical disc based on the TE1 signal and the TE2 signal.

[0037] In one embodiment of the invention, the optical disc apparatusfurther includes a second light distribution section for directing thetransmission light toward the transmission light detection section, anddirecting the first diffraction light and the second diffraction lighttoward the first diffraction light detection section.

[0038] In one embodiment of the.:invention, the transmission lightdetection section includes a first sub-transmission light detectionsection and a second sub-transmission light detection section. Firsttransmission light is defined as part of the transmission light, whichis detected by the first sub-transmission light detection section, andsecond transmission light is defined as a part of the transmissionlight, which is detected by the second sub-transmission light detectionsection. The offset of the transmission light is defined as a differencebetween a light amount of the first transmission light and a lightamount of the second transmission light.

[0039] In one embodiment of the invention, the first diffraction lightdetection section includes a first sub-diffraction light detectionsection for detecting the first diffraction light and a secondsub-diffraction light detection section for detecting the seconddiffraction light.

[0040] In one embodiment of the invention, the control device obtainsthe tracking error signal by TE2−k×TE1.

[0041] In one embodiment of the invention, the transmission lightdetection section includes a third area and a fourth area. The firstsub-transmission light detection section is provided in the third area,and the second sub-transmission light detection section is provided inthe fourth area. A border between the third area and the fourth area isparallel to a rotation direction of the optical disc.

[0042] In one embodiment of the invention, the first diffraction lightdetection section includes a fifth area and a sixth area. The firstsub-diffraction light detection section is provided in the fifth area,and the second sub-diffraction light detection section is provided inthe sixth area. A border between the fifth area and the sixth area isparallel to a rotation direction of the optical disc.

[0043] In one embodiment of the invention, the control device updates avalue of k in accordance with a logical product of a numerical aperture(NA) of the objective lens and a pitch (P) of the optical disc in adiameter direction of the optical disc (NA×P).

[0044] In one embodiment of the invention, a value of k is 0.5×S2/S1 orless, wherein S1 is a light amount of the transmission light detected bythe transmission light detection section, and S2 is a light amount ofthe diffraction light detected by the first diffraction light detectionsection.

[0045] In one embodiment of the invention, the control device sets thevalue of k at zero when the logical product of the numerical aperture(NA) of the objective lens and the pit pitch (P) of the optical disc inthe diameter direction of the optical disc (NA×P) is 0.9 times or moreof the wavelength of the light incident on the optical disk.

[0046] In one embodiment of the invention, the control device sets avalue of k so that an average output level of TE2−k×TE1 is substantiallyzero when the control device shifts the objective lens in a diameterdirection of the optical disc without performing tracking control.

[0047] In one embodiment of the invention, the optical disc apparatusfurther includes an aberration section for providing the transmissionlight with an aberration. The tranismission light detection sectionincludes a third area, a fourth area, a seventh area and an eighth area.The first sub-transmission light detection section is provided in thethird area. The second sub-transmission light detection section isprovided in the fourth area. The third sub-transmission light detectionsection is provided in the seventh area. The fourth sub-transmissionlight detection section is provided in the light area. A border betweenthe third area and the fourth area is parallel to a rotation directionof the optical disc. A border between the third area and the eighth areais parallel to a diameter direction of the optical disc. A borderbetween the fourth area and the seventh area is parallel to a diameterdirection of the optical disc. A border between the seventh area and theeighth area is parallel to a rotation direction of the optical disc. Thethird area is orthogonal with respect to the seventh area. The fourtharea is orthogonal with respect to the eighth area. The control deviceobtains a focusing error signal for the optical disc based on adifference between a sum of a light amount of the transmission lightprovided with the aberration and detected by the first sub-transmissionlight detection section and a light amount of the transmission lightprovided with the aberration and detected by the third sub-transmissionlight detection section, and a sum of a light amount of the transmissionlight provided with the aberration and detected by the secondsub-transmission light detection section and a light amount of thetransmission light provided with the aberration and detected by thefourth sub-transmission light detection section.

[0048] In one embodiment of the invention, the first light distributionsection includes a ninth area and a tenth area. The first Lightdistribution section outputs the light reflected by the optical disc anddiffracted by the ninth area of the first light distribution section asthird diffraction light, and outputs the light reflected by the opticaldisc and diffracted by the tenth area of the first light distributionsection as fourth diffraction light. The first diffraction lightdetection section includes a first sub-diffraction light detectionsection, a second sub-diffraction light detection section, a thirdsub-diffraction light detection section: a fourth sub-diffraction lightdetection section, a fifth sub-diffraction light detection section, anda sixth sub-diffraction light detection section. The first diffractionlight is detected by the first sub-diffraction detection section and thesecond sub-diffraction detection section. The second diffraction lightis detected by the fifth sub-diffraction detection section and the sixthsub-diffraction detection section. The third diffraction light isdetected by the fourth sub-diffraction detection section and the fifthsub-diffraction detection section. The fourth diffraction light isdetected by the second sub-diffraction detection section and the thirdsub-diffraction detection section. The control device obtains a focusingerror signal for the optical disc based on a difference between a totallight amount of the diffraction light detected by the firstsub-diffraction light detection section, the third sub-diffraction lightdetection section and the fifth sub-diffraction light detection section,and a total light amount of the diffraction light detected by the secondsub-diffraction light detection section, the fourth sub-diffractionlight detection section and the sixth sub-diffraction light detectionsection.

[0049] In one embodiment of the invention, the optical disc apparatusfurther includes a second diffraction light detection section. The firstlight distribution section outputs the light, reflected by the optical,disc and diffracted by the first area of the first light distributionsection separately from the first diffraction light, as fifthdiffraction light, and outputs the light, reflected by the optical discand diffracted by the second area of the first light distributionsection separately from the second diffraction light, as sixthdiffraction light. The second diffraction light detection sectionincludes a seventh sub-diffraction light detection section and an eighthsub-diffraction light detection section. The control device obtains afocusing error signal for the optical disc based on a difference betweena light amount of the fifth diffraction light detected by the seventhsub-diffraction light detection section and alight amount of the sixthsub-diffraction light detected by the eighth sub-diffraction lightdetection section.

[0050] In one embodiment of the invention, the first light distributionsection includes a holographic element having a pattern havingsawtooth-lie or step-like shape including three or more steps, thepattern being continuous over sequential cycles. The first lightdistribution section outputs the light, reflected by the optical discand diffracted by the first area of the first light distribution sectionseparately from the first diffraction light, as fifth diffraction light,and outputs the light, reflected by the optical disc and diffracted bythe second area of the first light distribution section separately fromthe second diffraction light, as sixth diffraction light. A light amountof the first diffraction light and a light amount of the fifthdiffraction light both output by the first light distribution sectionare different from each other, and a light amount of the seconddiffraction light and a light amount of the sixth diffraction light bothoutput by the first light distribution section are different from eachother.

[0051] In one embodiment of the invention, the first diffraction lightand the second diffraction light output by the first light distributionsection are positive first order diffraction light, and the fifthdiffraction light and the sixth diffraction light output by the firstlight distribution section are negative first order diffraction light.

[0052] In one embodiment of the invention, a light amount of thenegative first order diffraction light is substantially zero.

[0053] In one embodiment of the invention, a light amount output by thefirst light distribution section is largest for the positive first orderdiffraction light, second largest for the transmission light, andsmallest for the negative first order diffraction light.

[0054] In one embodiment of the invention, a light amount output by thefirst light distribution section is largest for the transmission light,second largest for the positive first order diffraction light, andsmallest for the negative first order diffraction light.

[0055] In one embodiment of the invention, a light amount output by thefirst light distribution section is largest for the transmission light,second largest for the negative first order diffraction light, andsmallest for the positive first order diffraction light.

[0056] In one embodiment of the invention, the optical disc apparatusfurther includes a second diffraction light detection section. The firstlight distribution section includes a ninth area and a tenth area. Thefirst light distribution section outputs the light reflected by theoptical disc and diffracted by the ninth area of the first lightdistribution section as third diffraction light, outputs the lightreflected by the optical disc and diffracted by the tenth area of thefirst light distribution section as fourth diffraction light, outputsthe light, reflected by the optical disc and diffracted by the firstarea of the first light distribution section separately from the firstdiffraction light, as fifth diffraction light, and outputs the light,reflected by the optical disc and diffracted by the second area of thefirst light distribution section separately from the second diffractionlight, as sixth diffraction light. The second diffraction lightdetection section includes an eleventh area, a twelfth area, athirteenth area, a fourteenth area, a fifteenth area, and a sixteentharea. A seventh sub-diffraction light detection section is provided inthe eleventh area. An eighth sub-diffraction light detection section i sprovided in the twelfth area. A ninth sub-diffraction light detectionsection is provided in the thirteenth area. A tenth sub-diffractionlight detection section is provided in the fourteenth area. An eleventhsubsidization light detection section is provided in the fifteenth Area.A twelfth sub-diffraction light detect Ion sect Ion is provided in thesixteenth area. The third diffraction light lo detected by the seventhsub-diffraction light detection section and the eighth sub-diffractionlight detection section. The fourth diffraction light is detected by theis eleventh sub-diffraction light detection section and the twelfthsub-diffraction light detection section. The fifth diffraction light isdetected by the tenth sub-diffraction light detection section and theeleventh sub-diffraction light detection section. The sixth diffractionlight is detected by the eighth sub-diffraction light detection sectionand the ninth sub-diffraction light detection section. The controldevice obtains a focusing error signal for the optical disc based on adifference between a total light amount of the diffraction lightdetected by the seventh sub-diffraction light detection section, theninth sub-diffraction light detection section and the eleventhsub-diffraction light detection section, and a total light amount of thesub-diffraction light detected by the eighth sub-diffraction lightdetection section, the tenth sub-diffraction light detection section andthe twelfth sub-diffraction light detection section.

[0057] In one embodiment of the invention, the optical disc apparatusfurther includes a second diffraction light detection section. The firstlight distribution section includes a ninth area and a tenth area. Thefirst light distribution section outputs the light reflected by theoptical disc and diffracted by the ninth area of the first lightdistribution section as third diffraction light, outputs the lightreflected by the optical disc and diffracted by the tenth area of thefirst light distribution section as fourth diffraction light, outputsthe light, reflected by the optical disc and diffracted by the firstarea of the first light distribution section separately from the firstdiffraction light, as fifth diffraction light, and outputs the light,reflected by the optical disc and diffracted by the second area of thefirst light distribution section separately from the second diffractionlight, as sixth diffraction light. The second diffraction lightdetection section includes an eleventh area, a twelfth area, athirteenth area, a fourteenth area, a fifteenth area, and a sixteentharea. A seventh sub-diffraction light detection section is provided inthe eleventh area. An eighth sub-diffraction light detection section isprovided in the twelfth area. A ninth sub-diffraction light detectionsection is provided in the thirteenth area. A tenth sub-diffractionlight detection section is provided in the fourteenth area. An eleventhtenth sub-diffraction light detection section is provided in thefifteenth area. A twelfth sub-diffraction light detection section isprovided in the sixteenth area. The third diffraction light is detectedby the seventh sub-diffraction light detection section and the eighthsub-diffraction light detection section. The fourth diffraction light isdetected by the eighth sub-diffraction light detection section and theninth sub-diffraction light detection section. The fifth diffractionlight is detected by the tenth sub-diffraction light detection sectionand the eleventh sub-diffraction light detection section. The sixthdiffraction light is detected by the eleventh sub-diffraction lightdetection section and the twelfth sub-diffraction light detectionsection. The control device obtains a focusing error signal for theoptical disc based on a difference between a total light amount of thediffraction light detected by the seventh sub-diffraction lightdetection section, the ninth sub-diffraction light detection section andthe eleventh sub-diffraction light detection section, and a total lightamount of the sub-diffraction light detected by the eighthsub-diffraction light detection section, the tenth sub-diffraction lightdetection section, and the twelfth sub-diffraction light detectionsection.

[0058] Thus, the invention described herein makes possible theadvantages of providing an optical disc apparatus for sufficientlydecreasing the degree of asymmetry of a tracking error signal caused bythe shift of the central axis of an objective lens with respect to theoptical axis of the optical disc apparatus and suppressing off-track, soas to realize satisfactory and stable recording and reproduction.

[0059] These and other advantages of the present invention will becomeapparent to those Skilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060]FIG. 1A is a schematic view of an optical disc apparatus accordingto a first example of the present invention;

[0061]FIG. 1B shows a structure of a polarizing holographic face in theoptical disc apparatus of the first example:

[0062]FIG. 1C shows a structure of a photodetector in the optical discapparatus of the first example;

[0063]FIG. 2 is a contour diagram illustrating the degree of asymmetryof a TE2 signal in the optical disc apparatus of the first example whenlaser light crosses pits of an optical disc (pit pitch p=1.23 μm);

[0064]FIG. 3 is a graph illustrating the diffraction light amount ratiosof a polarizing holographic element in the optical disc apparatus of thefirst example;

[0065]FIG. 4A is a schematic view of an optical disc apparatus accordingto a second example of the present invention;

[0066]FIG. 4B shows a structure of a photodetector in the optical discapparatus of the second example;

[0067]FIG. 5A shows a structure of a polarizing holographic face in anoptical disc apparatus according to a third example;

[0068]FIG. 5B shows a structure of a photodetector in the optical discapparatus of the third example;

[0069]FIG. 6A shows a structure of a polarizing holographic face in anoptical disc apparatus according to a fourth example of the presentinvention;

[0070]FIG. 6B shows a structure of a photodetector in the optical discapparatus of the fourth example;

[0071]FIG. 7A shows a structure of a polarizing holographic face in anoptical disc apparatus according to a fifth example of the presentinvention:

[0072]FIG. 7B shows a structure of a photodetector in the optical discapparatus of the fifth example;

[0073]FIG. 8A shows a structure of a polarizing holographic face in anoptical disc apparatus according to a sixth example of the presentintention;

[0074]FIG. 8B shows a structure of a photodetector in the optical discapparatus of the sixth example;

[0075]FIG. 9A shows a structure of a polarizing holographic face in anoptical disc apparatus according to a seventh example of the presentinvention;

[0076]FIG. 9B shows a structure of a photodetector in the optical discapparatus of the seventh example;

[0077]FIG. 10A is a schematic view of a first conventional optical discapparatus;

[0078]FIG. 10B shows a structure of a photodetector in the firstconventional optical disc apparatus:

[0079]FIG. 11A is a schematic view of a second conventional optical discapparatus;

[0080]FIG. 11B shows a structure of a polarizing holographic face in thesecond conventional optical disc apparatus:

[0081]FIG. 11C shows a structure of a photodetector in the secondconventional optical disc apparatus:

[0082]FIG. 12 show light intensity distributions in a cross-sectionalong a diameter direction of an optical disc when a central axle of anobjective lens is shifted with respect to an optical axis of the opticaldisc apparatus:

[0083]FIG. 13A it a contour diagram illustrating the degree of asymmetryof a TE1 signal in the first conventional optical disc apparatus (pitpitch p=0.74 μm):

[0084]FIG. 13B is a signal waveform diagram illustrating asymmetry of asignal; and

[0085]FIG. 14 is a contour diagram illustrating the degree of asymmetryof a TE2 signal in the second conventional optical disc apparatus (pitpitch p=0.74 μm).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0086] Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

EXAMPLE 1

[0087] An optical disc apparatus 100 according to a first example of thepresent invention will be described with reference to FIGS. 1A through1C, 2, 3, 13A, 13B and 14.

[0088]FIG. 1A shows the optical disc apparatus 100. Laser light emittedby a laser light source 110 is converged on an optical disc 170 throughan optical system 115. The light reflected by the optical disc 170 isdetected by a photodetector 200. Based on a result detected by thephotodetector 200, a control device 185 controls an element or elementsamong the light source 110, the optical system 115, and the optical disc170 as necessary. The optical system 115 includes, for example, apolarizing beam slitted 120 having a splitting face 125, a collimatorlens 130, a quarter-wave plate 142, a reflecting mirror 140, apolarizing holographic element 145, and an objective lens 160.

[0089] A more specific operation of the optical disc apparatus 100 willbe described.

[0090] Laser light emitted by the light source 110 is incident on thepolarizing beam splitter 120 and transmitted through the splitting face125 of the polarizing beam splatter 120, and then converted intoparallel light by the collimator lens 130. The light source 110 is, forexample, a semiconductor laser. The parallel light is reflected by thereflecting mirror 140 and incident on the polarizing holographic element145.

[0091] The polarizing holographic element 145 is integrated into a lensholder 165 together with the objective lens 160. The polarizingholographic element 145 has the quarter-wave plate 142. A surface of thepolarizing holographic element 145 is a polarizing holographic face 150.

[0092] The light (P wave), which in incident on the polarizingholographic element 145, is transmitted through the polarizingholographic face 150 and converted into circular polarization by thequarterwave plate 142, collected by the objective lens 160, and thenconverged on a signal face 174 of the optical disc 170.

[0093] The optical disc 170 has the signal face 174 between a substrate172 and a protection film 176. The signal face 174 has pits (or grooves)formed in a rotation direction of the optical disc 170. The pits eachhave a depth d and a width w, and arranged at a pitch p.

[0094] The light reflected by the signal face 174, which is circularlypolarized, to transmitted through the objective lens 160, converted intolinear polarization (S wave) by the quarter-wave plate 142, and thendiffracted by or transmitted through the polarizing holographic face150. In this specification, 0th order diffraction is defined to betransmission. Then, the light is reflected by the reflecting mirror 140,made convergent by the collimator lens 130, reflected by the splittingface 125 of the polarizing beam splitter 120, and then collected on thephotodetector 200 as light 180. Based on a signal detected by thephotodetector 200, the control device 185 controls an element orelements among the light source 110, the optical system 115, and theoptical disc 170 as necessary. The photodetector 200 detects, forexample, a focusing error signal or a tracking error signal for theoptical disc 170.

[0095] In this specification, a holographic element acts as a firstlight distribution section, and a polarizing beam splitter acts as asecond light distribution section.

[0096]FIG. 1B shows a structure of the polarizing holographic face 150.The polarizing holographic face 150 includes two areas 150 a and 150 bwhich are separated from each other by a separation line 152. The areas150 a and 150 b have different holographic patterns. The separation line152 is parallel to a rotation direction of the optical disc 170. Thelight reflected by the optical disc 170 (i.e., a light beam 151) issubstantially equally divided into two by the separation line 152. Thetransmission light (0th order light) or diffraction light (for example,1st order light) passing through the polarizing holographic face 150 isreflected by the reflecting mirror 140 and made convergent by thecollimator lens 130. Then, the light is reflected by the splitting face125 of the polarizing beam splitter 120 and collected on thephotodetector 200 as the light 180.

[0097]FIG. 1C shows a structure of the photodetector 200. Thephotodetector 200 includes a transmission light detector 210 fordetecting transmission light, and a first diffraction light detector 220and a second diffraction light detector 230 both for detectingdiffraction light. The transmission light detector 210 is provided in acentral area of the photodetector 200. The first diffraction lightdetector 220 and the second diffraction light detector 230 are providedin a first outer area and a second outer area, respectively, of thephotodetector 200 so as to interpose the transmission light detector 210therebetween.

[0098] The transmission light detector 210 includes foursub-transmission light detectors 210A1, 210A2, 210B1 and 210B2. Thetransmission light detector 210 includes four areas 210C1, 210C2, 210C3and 210C4. The sub-transmission light detector 210A1 is provided in thearea 210C1. The sub-transmission light detector 210A2 is provided in thearea 210C2. The sub-transmission light detector 210B1 lo provided in thearea 210C3. The sub-transmission light detector 210B2 is provided in thearea 210C4. The areas 210C1, 210C2, 210C3 and 210C4 are separated fromeach other by separation lines 211 and 212 which are perpendicular toeach other. The separation line 211 extends parallel to the rotationdirection of the optical disc 170.

[0099] The first diffraction light detector 220 provided in the firstouter area includes two sub-diffraction light detectors 220A and 220B.The first diffraction light detector 220 includes areas 220C1 and 220C2.The sub-diffraction light detector 220A is provided in the area 220C1The sub-diffraction light detector 220B is provided in the area 220C2.

[0100] The second diffraction light detector 230 provided in the secondouter area includes two sub-diffraction light detectors 230A and 230B.The second diffraction light detector 230 includes areas 230C1 and230C2. The sub-diffraction light detector 230A is provided in the area230C1. The sub-diffraction light detector 230B is provided in the area230C2.

[0101] Positive first order diffraction light diffracted by the area 150a of the polarizing holographic face 150 is collected on thesub-diffraction light detector 220A as a spot 182 a. Negative firstorder diffraction light diffracted by the area 150 a of the polarizingholographic face 150 (FIG. 1B) is focused after the sub-diffractionlight detector 230A and collected on the sub-diffraction light detector230A as a spot 183 a.

[0102] Positive first order diffraction light diffracted by the area 150b of the polarizing holographic face 150 (FIG. 1B) is collected on thesub-diffraction light detector 220B as a spot 182 b. Negative firstorder diffraction light diffracted by the area 150 b of the polarizingholographic face 150 is focused before the sub-diffraction lightdetector 230B and collected on the sub-diffraction light detector 230Bas a spot 183 b. The light transmitted through the polarizingholographic face 150 (0th order light or transmission light) iscollected substantially at an intersection of the separation lines 211and 212 of the transmission light detector 210 (in a central area of thetransmission light detector 210) as a spot 181. This light is focusedafter the detection face of the transmission light detector 210.

[0103] The sub-diffraction light detectors 220A and 220B of the firstdiffraction light detector 220 each detect a light amount. A secondtracking error signal 2435 (TE2 signal) is obtained by subjecting thedetected light amounts to a subtraction performed by a subtracter 243. Areproduction signal 244 s is obtained by subjecting the detected lightamounts to addition performed by an adder 244. The TE2 signalcorresponds to the TE2 signal detected by the photodetector 1190 shownin FIG. 1C.

[0104] Based on detection results of the sub-transmission lightdetectors 210A1, 210A2, 210B1 and 210B2, a calculator 241 of thephotodetector 200 outputs 210A1+210A2−210B1−210B2. The output from thecalculator 241 is a first tracking error signal 241 s (TE1 signal). TheTE1 signal corresponds to the TE1 signal detected by the photodetector1050 shown in FIG. 10B. Also based on detection results of thesub-transmission light detectors 210A1, 210A2, 210B1 and 210B2, acalculator 242 of the photodetector 200 outputs 210A1+210B2−210A2−210B1.The output from the calculator 242 is a third tracking error signal 242s (TE3 signal) The TE3 signal is generally referred to as a phasedifferential TE (tracking error) signal.

[0105] In this example, the transmission light detector 210, which issubstantially rectangular, is divided into sub-transmission lightdetectors 210A1, 210A2, 210B1 and 210B2, which are also substantiallyrectangular, in this cases the difference between the light amountdetected by two sub-transmission light detectors adjacent in a directionparallel to the rotation direction of the optical disc 170 (210A1 and210A2) and the light amount detected by the other two sub-transmissionlight detectors (210B1 and 210B2) is the TE1 signal. The differencebetween the light amount detected by two sub-transmission lightdetectors orthogonally provided (210A1 and 210B2) and the light amountdetected by the other two sub-transmission light detectors (210A2 and210B1) is the TE3 signal.

[0106] The sub-diffraction light detectors 230A and 230B of the seconddiffraction light detector 230 each detect a light amount. A focusingerror signal 245 s (FE signal) is obtained by subjecting the detectedlight amounts to subtraction performed by a subtracter 245.

[0107] The control device 185 generates a tracking error signal for theoptical disc 170 based on the TE1 and TE2 signals.

[0108] In this example, three types of tracking error signals (TE1, TE2and TE3 signals) are obtained. These tracking error signals can be usedin accordance with the type of the optical disc. For example, in thecase of an optical disc having a pit depth corresponding to about ¼ ofthe wavelength (e.g., DVD-ROM disc), the control device 185 can use aTE3 signal as a tracking error signal with respect to a pit signal(emboss signal).

[0109] In the case of an optical disc having a guide groove such as forexample, a DVD-RAM disc or DVD-R disc, the control device 185 can use acalculation result value of TE2−k×TE1, obtained by using an appropriateconstant k, as a tracking error signal, in this case, the control device185 can update the value of k in accordance with the type of the opticaldisc.

[0110] For example, in the case where the optical disc 170 has a pitpitch of 0.74 μm, the TE1 signal shows asymmetry as shown in FIG. 13Afor the reason described regarding the photodetector 1050 (FIG. 10B)when the objective lens 160 is shifted in the direction of arrow K (FIG.1A). The TE2 signal also shows the asymmetry as shown in FIG. 14 for thereason described regarding the photodetector 1190 (FIG. 11C).Accordingly, where the shifting amount of the objective lens 160 is X,the level of a true tracking error signal (tracking error signal with noinfluence of the shifting of the objective lens 160) is TE, the totallight amount received by the transmission light detector 210 is S1, andthe total light amount received by the first diffraction light detector220 is S2, the following expressions can be provided.

TE 1/S 1=TE+X  expression 1

TE 2/S 2=TE+m×X   expression 2

[0111] At point R (where the width w of the pits is 0.30 μm and thedepth of the pits is λ/10), coefficient m=0.18/0.52=1/2.89. At point R′(where the width w of the pits is 0.34 μm and the depth of the pits isλ/12), coefficient m=0.22/0.62=1/2.82. At points other than point R, mis in the vicinity of 1/2.89 (see FIGS. 13A and 14).

[0112] From expressions 1 and 2, expression 3 is obtained.

TE=(TE 2−k×TE 1)/S 2(1−m)  expression 3

[0113] where k is given by expression 4.

k=m×S 2/S 1  expression 4

[0114] When the pit pitch P of the optical disc 170 is 0.74 μm, atracking error signal with no influence of the shifting of the objectivelens 160 is obtained by using, as the tracking error signal, thecalculation result of TE2−k×TE1 with k fulfilling expression 4. In thismanner, the degree of asymmetry of the tracking error signal caused bythe shifting of the objective lens 160 can be suppressed.

[0115]FIG. 2 is a graph illustrating the degree of asymmetry of thewaveform of the TE2 signal when the laser light crosses the pit a (whentracking is off). The optical disc has a pit pitch of 1.23 μm. Thedegree of asymmetry is represented as contours. The other conditions arethe same as those of FIG. 13A. At point S (the width w of the pits is0.615 μm and the depth of the pits is λ/12), the degree of asymmetry ofthe TE2 signal is 0.00. Even at points shifted from point S in the pitdepth and pit width, the degree of asymmetry of the TE2 signal is almostzero. This is because when the pit pitch p=1.23 μm, the light intensitydistributions 1233 (part (c) of FIG. 12) and 1234 (part (d) of FIG. 12)are almost uniform, and thus the patterns ABC′D′ and C′D′EF have almostequal areas to each other.

[0116] Accordingly, in the case where the pit pitch of the optical discis 1.23 μm, when the control device 185 sets k=0 the calculated level ofthe TE signal (TE2−k×TE1) is equal to that of the TE2 signal. The TEsignal is not influenced by the shifting of the objective lens and thedegree of asymmetry of the TE signal is sufficiently suppressed.

[0117] Therefore, in the case where the optical disc 170 has arelatively large pit pitch, such as a DVD-RAM disc or the like, thecontrol device 185 sets k=0. In the case where the optical disc 170 hasa relatively small pit pitch as a DVD-R disc, a DVD-RW disc or the like,the control device 185 sets k=m×S2/S1. The value of m is a constantvalue in the range of, for example, ½ to ⅕. The optimum value of m canbe determined in accordance with the pit pitch of the optical disc 170,the numerical aperture (NA) of the objective lens 160, the ratio of therim intensity of the light incident on the objective lens 160 (i.e., theratio of the light intensity at the rim of the objective lens 160 withrespect to the peak light intensity) or the like. The update of theconstant k performed by the control device 185 can be determined inaccordance with whether or not the logical product of the numericalaperture (NA) of the objective lens 160 and the pit pitch (P) of theoptical disc 170 in the diameter direction thereof (NA×P) is larger thana prescribed value (for example, 0.9 times the wavelength).

[0118] By switching the value of k as described above, the degree ofasymmetry of the TE signal caused by the shifting of the objective lens160 is sufficiently suppressed even when a different type of opticaldisc is mounted. Off-track while the tracking control is performed canbe solved. The update of the value of k can be performed a plurality oftimes in accordance with the pitch of the optical disc, instead of onceas in the above-described example. The optimum value of k can bedetermined by learning. In this case, the control device 185 can set thevalue of constant k so that the average output level of the calculatedsignal TE2−k×TE1 (average value of the maximum value and the minimumvalue of the calculated signal) obtained when the objective lens 160 isshifted in the diameter direction of the optical disc 170-withouttracking control is almost zero (ground level).

[0119]FIG. 3 is a graph illustrating the diffraction light amount ratioof the polarizing holographic element 145. The polarizing holographicface 150 of the polarizing holographic element 145 does notsubstantially diffract the light propagating toward the optical disc 170(P wave) but diffracts the light propagating from the optical disc 170(S wave). FIG. 3 also shows a phase distribution 19 of the wave surfaceof the light immediately after being transmitted through the polarizingholographic face 150. The phase distribution 19, or the holographicpattern, has a sawtooth-like or step-like shape, the pattern beingcontinuous over sequential cycles. A first step 19 a, a second step 19 band a third step 19 c, each of which corresponds to one cycle of phase,have width ratios of 37%, 25% and 38%, respectively. A phase differencebetween the first step 19 a and the second step 19 b and the phasedifference between the second step 19 b and the third step 19 a are each75 degrees.

[0120] Due to such a cyclic step-like phase distribution 19, diffractionlight is generated. Where the total of the transmission light and thediffraction light is 100% the ratio of the 0th order light amount(transmission light amount) is 20%, the ratio for the positive firstorder diffraction light amount is 47.6%, and the ratio for the negativefirst order diffraction light amount is 12.4%. The rest is allocated tohigher order diffraction light The optical disc apparatus 100 in thefirst example generates a reproduction signal using positive first orderdiffraction light 182 a and 182 b (FIG. 1C) detected by thesub-diffraction light detectors 220A and 220B. Accordingly, when theratio of the positive first diffraction light amount is higher as shownin FIG. 3, a signal having a relatively high S/N ratio can be generated.Generally, the S/N ratio is in proportion to the detection index(detected light amount/{square root} (number of sub detectors fordetecting the light)). In this example, the detection index=47.6/{squareroot}{square root over (2)}=34. The phase differential TE signal (TE3signal) with respect to the pit signal (emboss signal) generallyrequires high frequency signal processing, but does not involve anyproblem in terms of the S/N ratio since the ratio of the 0th order lightis about 20%.

[0121] In the optical disc apparatus 100 in the first example, the lightsource 110 and the photodetector 200 are separately provided, unlike inthe conventional optical disc apparatus 1100. Therefore, thetransmission light can be used in order to obtain a tracking errorsignal. The optical apparatus 100 in the first example, includes thepolarizing beam aplitter 120, but those stilled in the art would readilyconceive various structures without the polarizing beam splitter 120.

[0122] In the optical disc apparatus 100 in the first example, the lightemitted by the light source 110 is diffracted after being reflected bythe optical disc 170. Therefore, the light can be efficiently incidenton the optical disc apparatus 200.

[0123] In the above description, ±1st order diffraction light is used asthe diffraction light. Higher order diffraction light (e.g., ±2nd or 3rdorder diffraction light) can be used. The spot 181 can be focused beforethe detection face of the transmission light detector 210. In this case,the light distribution is inverted with respect to the optical axis, andthus the polarity of the TE1 signal is changed. This can be handled bychanging “TE1” in the above description into “−TE1”. The same effect asdescribed is provided.

EXAMPLE 2

[0124]FIG. 4A schematically shows an optical disc apparatus 300according to a second example of the present invention. The optical discapparatus 300 has the same structure as that of the optical discapparatus 100 in the first example except that a parallel flat plate 370is provided between the polarizing beam splitter 120 and a photodetector400 and that the photodetector 400 had a different structure from thatof the photodetector 200. Identical elements, to those of the firstexample will bear identical reference numeral and will not be describedin detail. The parallel flat plate 370 is provided inclined withrespected to an optical axis of converged light 380 incident on theparallel flat plate 370. By this inclination, the light passing throughthe parallel flat plate 370 is provided with aberration (astigmatism) bywhich focal lines extending in ±45 degree directions with respect to aseparation line 411 (FIG. 4B) appears on a detection face of thephotodetector 400. The parallel flat plate 370 acts as an aberrationsection.

[0125]FIG. 4B shows the photodetector 400. The photodetector 400includes a transmission light detector 410 and a diffraction lightdetector 420.

[0126] The transmission light detector 410 includes foursub-transmission light detectors 410A1, 410A2, 410B1 and 410B2. Thetransmission light detector 410 includes four areas 410C1, 410C2, 410C3and 410C4. The sub-transmission light detector 410A1 is provided in thearea 410C1. The sub-transmission light detector 410A2 is provided in thearea 410C2. The sub-transmission light detector 410B1 is provided in thearea 410C3. The sub-transmission light detector 410B2 is provided in thearea 410C4. The areas 410C1, 410C2, 410C3 and 410C4 are separated fromeach other by separation lines 411 and 412 which are perpendicular toeach other. The separation line 411 extends parallel to the rotationdirection of the optical disc 170.

[0127] The diffraction light detector 420 includes two sub-diffractionlight detectors 420A and 420B. The diffraction light detector 420includes areas 420C1 and 420C2. The sub-diffraction light detector 420Ais provided in the area 420C1. The sub-diffraction light detector 420Bis provided in the area 420C2.

[0128] Positive first order diffraction light diffracted by the area 150a of the polarizing holographic face 150 (FIG. 1B) is focused before thesub-diffraction light detector 420A and collected on the sub-diffractionlight detector 420A as a spot 382 a. Positive first order diffractionlight diffracted by the area 150 b of the polarizing holographic face150 is focused after the sub-diffraction light detector 420B andcollected on the sub-diffraction light detector 420B as a spot 382 b. Inthis example, whether the focal point is before or after the detectionface does not matter. The focal point can be before or after thedetection face.

[0129] The light transmitted through the polarizing holographic face 150(0th order light or transmission light) is collected substantially at anintersection of the separation lines 411 and 412 of the transmissionlight detector 410 (in a central area of the transmission light detector410) as a spot 381. In this case, the detection face of the transmissionlight detector 410 is substantially at a mid point between two focallines (vertical focal line and horizontal fool line). Accordingly, whenthe spot 381 passe a focal line inclined clockwise at 45 degrees withrespect to the separation line 412 before reaching the detection face ofthe transmission light detector 410, the light distribution is symmetricwith respect to the focal line. The light distribution of the spot 381is equivalent to the light distribution which is rotated clockwise at 90degrees from that of the spot 181 in the first example.

[0130] The sub-diffraction light detectors 420A and 420B of thediffraction light detector 420 each detect a light amount. A secondtracing error signal 443 a (TE2 signal) is obtained by subjecting thedetected light amounts to subtraction performed by a subtracter 443. Areproduction signal 4448 is obtained by subjecting the detected lightamounts to addition performed by an adder 444. The TE2 signalcorresponds to the TE2 signal detected by the photodetector 1190 shownin FIG. 11.

[0131] Based on detection results of the sub-transmission lightdetectors 410A1, 410A2, 410B1 and 410B2, a calculator 441 of thephotodetector 400 outputs 410A1−410A2+410B1−410B2. The output from thecalculator 441 is a first tracking error signal 441. (TE1 signal). TheTE1 signal corresponds to the TE1 signal detected by the photodetector1050 shown in FIG. 10B. Also based on detection results of thesub-transmission light detectors 410A1, 410A2, 410B1 and 410B2, acalculator 442 of the photodetector 400 outputs 410A1+410B2−410A2−410B1.The output from the calculator 442 is a third tracking error signal 442s (TE3 signal).

[0132] Like in the first example, the transmission light detector 410,which is substantially rectangular, s divided into sub-transmissionlight detectors 410A1, 410A2, 410B1 and 410B2, which are alsosubstantially rectangular. In this case, the difference between thelight amount detected by two sub-transmission light detectors adjacentin a direction parallel to the rotation direction of the optical disc170 (410A1 and 410B1) (as described above, the light distribution isrotated clockwise at 90 degrees with respect to the light distributionin the first example, and therefore the separation line (412) parallelto the rotation direction of the optical disc 170 in the second exampleis also rotated at 90 degrees with respect to such a separation line(211) in the first example), and the light amount detected by the othertwo sub-transmission light detectors (410A2 and 410B2) is the TE1signals The difference between the light amount detected by twosub-transmission light detectors orthogonally provided (410A1 and 410B2)and the light amount detected by the other two sub-transmission lightdetectors (410A2 and 410B1) is the TE3 signal.

[0133] A focusing error of the objective lens 360 is reflected as anastigmatism of the converged light 381 (difference between ±45 degreedirections). Therefore, the third tracking error signal 442 s calculatedby the calculator 442 which outputs 410A1+410B2−410A2−410B1 correspondsto a focusing error signal (FE signal).

[0134] In this example also, three types of tracking error signals (TE1,TE2 and TE3 signals) are obtained. Like in the first example, thesetracking error signals can be used in accordance with the type of theoptical disc. For example, in the case of an optical disc having a pitdepth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc),the control device. 185 can use a TE3 signal as a tracking error signalwith respect to a pit signal (emboss signal).

[0135] In the case of an optical disc having a guide groove such as forexample, a DVD-RAM disc or DVD-R disc, the control device 185 can use acalculation result value of TE2−k×TE1, obtained by using an appropriateconstant k, as a tracking error signal. In this case, the control device185 can update the value of k in accordance with the type of the opticaldisc.

[0136] Like in the first example, the degree of asymmetry of thetracking error signal caused by the shifting of the central axis of theobjective lens 160 with respect to the optical axis of the optical discapparatus 300 can be sufficiently suppressed. Off-track while thetracking control is performed can be solved. In this example, negativefirst order diffraction light is not used. Therefore, thecross-sectional shape of the polarizing holographic element 145 can bechanged so as to eliminate the ratio of the negative first orderdiffraction light and thus increase the ratios of the 0th order andpositive first order diffraction light. In this manner, the S/N ratio ofthe reproduction signal and the phase differential TE signal (TE3signal) can be further improved compared to that of the first example.

[0137] As a modification of the second example, a sum of the lightamounts detected by the sub-transmission light detectors 410A1, 410A2,410B1 and 410B2 can be detected as a reproduction signal. In the casewhere the diffraction light ratios are 70% for the 0th order light and10% for the positive first diffraction light, the detection index of thereproduction signal is about 35. In this manner, the light amounts canbe adjusted so as to be largest for the transmission light, secondlargest for the positive first order diffraction light, and smallest forthe negative first order diffraction light.

[0138] In the above description, the parallel flat plate 370 is used asthe aberration section. The present invention is not limited to such astructure. For example, a wedge-like prism can be used as the aberrationsection.

EXAMPLE 3

[0139]FIG. 5A shows a structure of a polarizing holographic face 550 ofan optical disc apparatus according to a third example of the presentinvention. FIG. 5B shows a structure of a photodetector 500 of theoptical disc apparatus according to the third example of the presentinvention. The optical disc apparatus according to the third example hasthe same structure as that of the optical disc apparatus 100 in thefirst example except for the polarizing holographic face 550 and thephotodetector 500. The other elements will be described using thecorresponding reference numerals in FIG. 1A.

[0140] In FIG. 5A, the polarizing holographic face 550 is divided into afirst area 550 a, a second area 550 b, a third area 550 a and a fourtharea 550 d having different holographic patterns, along separation lines552 and 553. The separation line 552 is parallel to the rotationdirection of the optical disc 170, and the separation line 553 isperpendicular to the separation line 552. A light beam 551 reflected bythe optical disc 170 is substantially equally divided into four alongthe separation lines 552 and 553. The first area 550 a is furtherdivided into strip-shaped areas 550F11, 550B11, 550F12, 550B12 and550F13 along separation lines parallel to the separation line 553. Thesecond area 550 b is further divided into strip-shaped areas 550B21,550F21, 550B22, 550F22 and 550B23 along separation lines parallel to theseparation line 553. The third area 550 c is further divided intostrip-shaped areas 55031, 550B31, 550F32, 550B32 and 550F33 alongseparation lines parallel to the separation line 553. The fourth area550 d is further divided into strip-shaped areas 550B41, 550F41, 550B42,550F42 and 550B43 along separation lines parallel to the separation line553.

[0141] Negative first order diffraction light passing through thestrip-shaped areas having the letter “F” in their reference numerals(e.g., 550F11 or 550F22) is collected before the photodetector 500.Negative first order diffraction light passing through the strip-shapedareas having the letter “B” in their reference numerals (e.g., 550B11 or550B22) is collected after the photodetector 500.

[0142] Referring to FIG. 5B, the photodetector 500 includes atransmission light detector 510, a first diffraction light detector 520and a second diffraction light detector 530. The transmission lightdetector 510 is provided in a central area of the photodetector 500. Thefirst diffraction light detector 520 and the second diffraction lightdetector 530 are provided in a first outer area and a second outer area,respectively, of the photodetector 500 so as to interpose thetransmission light detector 510 therebetween.

[0143] The transmission light detector 510 includes foursub-transmission light detectors 510A1, 510A2, 510B1 and 510B2. Thetransmission light detector 510 includes four areas 510C1, 510C2, 510C3and 510C4. The sub-transmission light detector 510A1 is provided in thearea 510C1. The sub-transmission light detector 510A2 is provided in thearea 510C2. The sub-transmission light detector 510B1 is provided in thearea 510C3. The sub-transmission light detector 510B2 is provided in thearea 510C4. The areas 510C1, 510C2, 510C3 and 510C4 are separated fromeach other by separation lines 511 and 512 which are perpendicular toeach other. The separation line 511 extends parallel to the rotationdirection of the optical disc 170.

[0144] The first diffraction light detector 520 provided in the firstouter area includes two sub-diffraction light detectors 520A and 520B.The first diffraction light detector 520 includes areas 520C1 and 520C2.The sub-diffraction light detector 520A is provided in the area 520C1.The sub-diffraction light detector 520B is provided in the area 520C2.

[0145] The second diffraction light detector 530 provided in the secondouter area includes six sub-diffraction light detectors 530A1, 530A2,530A3, 530B1, 530B2 and 530B3. The sub-diffraction light detectors530A1, 530B2 and 530A3 are electrically conductive to each other. Thesub-diffraction light detectors 530B1, 530A2 and 530B3 are alsoelectrxically conductive to each other. The second diffraction lightdetector 530 includes areas 530C1, 530C2, 530C3, 530C4, 530C5 and 530C6.The sub-diffraction light detector 530A1 is provided in the area 530C1.The sub-diffraction light detector 530A2 is provided in the area 530C2.The sub-diffraction light detector 530A3 is provided in the area 530C3.The sub-diffraction light detector 530B1 is provided in the area 530C4.The sub-diffraction light detector 530B2 is provided in the area 530C5.The sub-diffraction light detector 530B3 is provided in the area 530C6.

[0146] Positive first order diffraction light diffracted by thestrip-shaped areas 550B11 and 550B12 of the first area 550 a of thepolarizing holographic face 550 (FIG. 5A; not adjacent to each other butinterposing the area 550F12 therebetween) is collected on thesub-diffraction light detector 520B as a spot 582B1. Negative firstorder diffraction light diffracted by the strip-shaped areas 50B11 and50B12 is collected on the sub-diffraction light detector 530B3 whilebeing also on the sub-diffraction light detector 530B2 as a spot 583B1.

[0147] Positive first order diffraction light diffracted by the otherstrip-shaped areas 550F11, 550F12 and 550F13 -is collected on thesub-diffraction light detector 520B as a spot 582F1. Negative firstorder diffraction light diffracted by the strip-shaped areas 550F11,550F12 and 550F13 is collected on the sub-diffraction light detector530B2 while being also on the sub-diffraction light detector 530B3 as aspot 583F1.

[0148] Positive first order diffraction light diffracted by thestrip-shaped areas 550B21, 550B22 and 550B23 of the second area 550 b(FIG. 5A; not adjacent to each other but interposing the areas 550F21and 550F22 therebetween) is collected on the sub-diffraction lightdetector 520A as a spot 582B2. Negative first order diffraction lightdiffracted by the strip-shaped areas 550B21, 550B22 and 550B23 iscollected on the sub-diffraction light detector 530A2 while being alsoon the sub-diffraction light detector 530A1 as a spot 583B2.

[0149] Positive first order diffraction light diffracted by the otherstrip-shaped areas 550F21 and 550F22 is collected on the sub-diffractionlight detector 520A as a spot 582F2. Negative first order diffractionlight diffracted by the strip-shaped areas 550F21 and 550F22 iscollected on the sub-diffraction light detector 530A1 while being alsoon the sub-diffraction light detector 530A2 as a spot 583F2.

[0150] Positive first order diffraction light diffracted by thestrip-shaped areas 550B31 and 550B32 of the third area 550 c (FIG. 5A;not adjacent to each other but interposing the area 550F32 therebetween)is collected on the sub-diffraction light detector 520A as a spot 582B3.Negative first order diffraction light diffracted by the strip-shapedareas 550B31 and 550B32 is collected on the sub-diffraction lightdetector 530A2 while being also on the sub-diffraction light detector530A3 as a spot 583B3.

[0151] Positive first order diffraction light diffracted by the otherstrip-shaped areas 550F31, 550F32 and 550F33 is collected on thesub-diffraction light detector 520A as a spot 582F3. Negative firstorder diffraction light diffracted by the strip-shaped areas 550F31,550F32 and 550F33 is collected on the sub-diffraction light detector530A3 while being also on the sub-diffraction light detector 530A2 as aspot 583F3.

[0152] Positive first order diffraction light diffracted by thestrip-shaped areas 550B41, 55042 and 550B43 of the fourth area 550 d(FIG. 5A: not adjacent to each other but interposing the areas 550F41and 550F42 therebetween) is collected on the sub-diffraction lightdetector 520B as a spot 582B4. Negative first order diffraction lightdiffracted by the strip-shaped areas 550B41, 550B42 and 550B43 iscollected on the sub-diffraction light detector 530B1 while being alsoon the sub-diffraction light detector 530B2 as a spot 583B4.

[0153] Positive first order diffraction light diffracted by the otherstrip-shaped areas 550F41 and 550F42 is collected on the sub-diffractionlight detector 520B as a spot 582F4. Negative first order diffractionlight diffracted by the strip-shaped areas 550F41 and 550F42 iscollected on the sub-diffraction light detector 530B2 while being alsoon the sub-diffraction light detector 530B1 as a spot 583F4.

[0154] The light transmitted through the polarizing holographic face 550(0th order light) is collected substantially at an intersection of theseparation lines 511 and 512 of the transmission light detector 510 (ina central area of the transmission light detector 510) as a spot 581.The focal point of the spot 581 is after the detection face of thetransmission light detector 510.

[0155] The sub-diffraction light detectors 520A and 520B of the firstdiffraction light detector 520 each detect a light amount. A secondtracking error signal 5438 (TE2 signal) is obtained by subjecting thedetected light amounts to a subtraction performed by a subtracter 543. Areproduction signal 544 s is obtained by subjecting the detected lightamounts to addition performed by an adder 544. The TE2 signalcorresponds to the TE2 signal detected by the photodetector 1190 shownin FIG. 11C.

[0156] The TE2 signal corresponds to a difference between the lightamount of the positive first order diffraction light diffracted by thefirst area 550 a and the fourth area 550 d of the polarizing holographicface 550 and the light amount of the positive first order diffractionlight diffracted by the second area 550 b and the third area 550 a ofthe polarizing holographic face 550. The reproduction signal correspondsto a sum of the light amount of the positive first order diffractionlight diffracted by the first area 550 a, the second area 550 b, thethird area 550 a and the fourth area 550 d.

[0157] Based on detection results of the sub-transmission lightdetectors 510A1, 510A2, 5101B and 510B2, a calculator 541 of thephotodetector 500 outputs 510A1+510A2−510B1−510B2. The output from thecalculator 541 is a first tracking error signal 541. (TE1 signal). TheTE1 signal corresponds to the TE1 signal detected by the photodetector1050 shown in FIG. 10B. Also based on detection results of thesub-transmission light detectors 510A1, 510A2, 51031 and 510B2, acalculator 542 of the photodetector 500 outputs 510A1+510B2−510A2−510B1.The output from the calculator 542 is a third tracking error signal 542s (TE3 signal).

[0158] In this example also, the transmission light detector 510, whichis substantially rectangular, is divided into sub-transmlssion lightdetectors 510A1, 510A2, 5101B and 510B2, which are also substantiallyrectangular. In this case, the difference between the light amountdetected by two sub-trenomission light detectors adjacent in a directionparallel to the rotation direction of the optical disc 170 (510A1 and510A2) and the light amount detected by the other two sub-transmissionlight detectors (510B1 and 5S0B2) in the TE1 signal. The differencebetween the light amount detected by two sub-transmission lightdetectors orthogonally provided (510A1 and 510B2) and the light amountdetected by the other two sub-transmlssion light detectors (510A2 and510B1) is the TE3 signal.

[0159] A calculator 545 outputs 530B1+530B3+530A2−530A1−530A3−530B2. Theoutput of the calculator 545 is a focusing error signal 545 s (FEsignal).

[0160] In this example also, three types of tracking error signals (TE1,TE2 and TE3 signals) are obtained. Like in the first example, thesetracking error signals can be used in accordance with the type of theoptical disc. For example, in the case of an optical disc having a pitdepth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc),the control device 185 can use a TE3 signal as a tracking error signalwith respect to a pit signal (emboss signal).

[0161] In the case of an optical disc having a guide groove such as, forexample, a DVD-RAM disc or DVD-R disc, the control device 185 can use acalculation result value of TE2−k×TE1 obtained, by using an appropriateconstant k, as a tracking error signal. In this case, the control device185 can update the value of k in accordance with the type of the opticaldisc.

[0162] Like in the first example, the degree of asymmetry of thetracking error signal caused by the shifting of the central axis of theobjective lens 160 with respect to the optical axis of the optical discapparatus 300 can be sufficiently suppressed. Off-track while thetracking control is performed can be solved. In this example, thepolarizing holographic face 550 is divided into small strip-shapedareas. Using these small strip-shaped areas, a light component to becollected before the photodetector 500 and a light component to becollected after the photodetector 500 are generated. The resultantdiffraction light is detected as an FE signal. Therefore, the adverseinfluence of dust and stains present on the substrate 172 of the opticaldisc 170 is negated. Thus, the focusing error control to highly stable.

[0163] In the above description, the sub-diffraction light detector530B1 is electrically conductive to the sub-diffraction light detectors530B3 and 530A2, and the sub-diffraction light detector 530B2 iselectrically conductive to the sub-diffraction light detector 530A1 and530A3. The difference between the outputs from the two groups of thesub-diffraction light detector is generated as an FE signal.Alternatively, the sub-diffraction light detectors 530B1 and 530B3,530A1 and 530A3 can be electrically conductive to each other, and thesub-diffraction light detector 530B2 can be electrically conductive tothe sub-diffraction light detector 530A2. In this case, an FE signal canbe generated by a difference signal thereof (i.e.,530B1+530B3+530A1+530A3−530B2−530A2). In this case on the seconddiffraction light detector 530, the spots 583B1 and 583F1 are exchangedwith the spots 583B4 and 583F4. Or on the second diffraction lightdetector 530, the spots 583B3 and 583F3 are exchanged with the spots58382 and 583F2. The spots on the first diffraction light detector 520are exchanged in correspondence therewith.

[0164] The polarizing holographic face 550 is not necessarily dividedinto the small strip-shaped areas. When the polarizing holographic face550 is not divided as shown in FIG. 5A, the first area 550 a and thethird area 550 a are entirely areas shown with “B”, and the second area550 b and the fourth area 550 d are entirely areas shown with “F”. Thespots 583F1, 583B2, 583F3 and 583B4 on the second diffraction lightdetector 530, and the spots 582F1, 582B2, 582F3 and 582B4 on the firstdiffraction light detector 520 are eliminated. Only the spots 583B1,583F2, 583B3 and 583F4 on the second diffraction light detector 530, andthe spots 582B1, 582F2, 582B3 and 582F4 on the first diffraction lightdetector 520 are left.

EXAMPLE 4

[0165]FIG. 6A shows a structure of a polarizing holographic face 650 ofan optical disc apparatus according to a fourth example of the presentinvention. FIG. 6B shows a structure of a photodetector 600 of theoptical disc apparatus according to the fourth example of the presentinvention. The optical disc apparatus according to the fourth examplehas the same structure as that of the optical disc apparatus t00 in thefirst example except for the polarizing holographic face 650 and thephotodetector 600. The other elements will be described using thecorresponding reference numerals in FIG. 1A.

[0166] In FIG. 6A, the polarizing holographic face 650 is divided into afirst area 650 a, a second area 650 b, a third area 650 c and a fourtharea 650 d having different holographic patterns, along separation lines652 and 653. The separation line 652 is parallel to the rotationdirection of the optical disc 170, and the separation line 653 isperpendicular to the separation line 652. A light beam 651 reflected bythe optical disc 170 is substantially equally divided into four alongthe separation line 652 and 653. The first area 650 a is further dividedinto strip-shaped areas 650F11, 650B11, 650F12, 650B12 and 650F13 alongseparation lines parallel to the separation line 653. The second area650 b is further divided into strip-shaped areas 650321, 650F21, 650B22,650F22 and 650B23 along separation lines parallel to the separation line653. The third area 650 c is further divided into strip-shaped areas650F31, 650B31, 650F32, 650B32 and 650F33 along separation linesparallel to the separation line 653. The fourth area 650 d is furtherdivided into strip-shaped areas 650B41, 650P41, 650B42, 650F42 and650B43 along separation lines parallel to the separation line 653.

[0167] Negative first order diffraction light passing through thestrip-shaped areas having the letter “F” in their reference numerals(e.g., 650F11 or 650F22) is collected before the photodetector 600.Negative first order diffraction light passing through the strip-shapedareas having the letter “B” in their reference numerals (e.g., 650B11 or650822) is collected after the photodetector 600.

[0168] Referring to FIG. 6B, the photodetector 600 includes atransmission light detector 610, a first diffraction light detector 620and a second diffraction light detector 630 the transmission lightdetector 610 is provided in a central area of the photodetector 600. Thefirst diffraction light detector 620 and the second diffraction lightdetector 630 are provided in a first outer area and a second outer area,respectively, of the photodetector 600 so as to interpose thetransmission light detector 610 therebetween.

[0169] The transmission light detector 610 includes foursub-transmission light detectors 610A1, 610A2, 6103B and 610B2. Thetransmission light detector 610 includes four areas 610C1, 610C2, 610C3and 610C4. The sub-transmission light detector 610A1 is provided in thearea S10C1. The sub-transmission light detector 610A2 is provided in thearea 610C2. The sub-transmission light detector 610B1 is provided in thearea 610C3. The sub-transmission light detector 610B2 is provided in thearea 610C4. The areas 610C1, 610C2, 610C3 and 610C4 are separated fromeach other by separation lines 611 and 612 which are perpendicular toeach other. The separation line 611 extends parallel to the rotationdirection of the optical disc 170.

[0170] The first diffraction light detector 620 provided in the firstouter area includes two sub-diffraction light detectors 620A and 620B.The first diffraction light detector 620 includes areas 620C1 and 620C2.The sub-diffraction light detector 620A is provided in the area 620C1The sub-diffraction light detector 620B is provided in the area 620C2.

[0171] The second diffraction light detector 630 provided in the secondouter area includes six sub-diffraction light detectors 630A1, 630A2,630A3, 630B1, 630B2 and 630B3. The sub-diffraction light detectors630A1, 630B2 and 630A3 are electrically conductive to each other. Thesub-diffraction light detectors 630B1, 630A2 and 630B3 are alsoelectrically conductive to each other. The second diffraction lightdetector 630 includes areas 630C1, 630C2, 630C3, 630C4, 630C5 and 630C6.The sub-diffraction light detector 630A1 is provided in the area 630C1.The sub-diffraction light detector 630A2 is provided in the area 630C2.The sub-diffraction light detector 630A3 is provided in the area 630C3.The sub-diffraction light detector 630B1 is provided in the area 630C4.The sub-diffraction light detector 630B2 is provided in the area 630C5.The sub-diffraction light detector 630B3 is provided in the area 630C6.

[0172] Positive first order diffraction light diffracted by thestrip-shaped areas 650B11 and 650B12 of the first area 650 a of thepolarizing holographic face 650 (FIG. 5A; not adjacent to each other butinterposing the area 650F12 therebetween) is collected on thesub-diffraction light detector 620B as a spot 682B1. Negative firstorder diffraction light diffracted by the strip-shaped areas 650B11 and650B12 is collected on the sub-diffraction light detector 630A2 whilebeing also on the sub-diffraction light detector 630A1 as a spot 683B1.

[0173] Positive first order diffraction light diffracted by the otherstrip-shaped areas 650F11, 650F12 and 650F13 is collected on thesub-diffraction light detector 6208 as a spot 682F1. Negative firstorder diffraction light diffracted by the strip-shaped areas 650F11,650F12 and 650F13 is collected on the sub-diffraction light detector630A1 while being also on the sub-diffraction light detector 630A2 as aspot 683F1.

[0174] Positive first order diffraction light diffracted by thestrip-shaped areas 650B21, 650B22 and 650B23 of the second area 650 b(FIG. 6A: not adjacent to each other but interposing the areas 650P21and 650P22 therebetween) is collected on the sub-diffraction lightdetector 620A as a spot 682B2. Negative first order diffraction lightdiffracted by the strip-shaped areas 650B21, 650B22 and 650B23 iscollected on the sub-diffraction light detector 630A3 while being alsoon the sub-diffraction light detector 630A2 as a spot 683B2.

[0175] Positive first order diffraction light diffracted by the otherstrip-shaped areas 650F21 and 650F22 is collected on the sub-diffractionlight detector. 620A as a spot 682F2. Negative first order diffractionlight diffracted by the strip-shaped areas 660F21 and 650P22 iscollected on the sub-diffraction light detector 630A2 while being alsoon the sub-diffraction light detector 630A3 as a spot 683F2.

[0176] Positive first order diffraction light diffracted by thestrip-shaped areas 650B31 and 650B32 of the third area 650 c (FIG. 6A:not adjacent to each other but interposing the area 650F32 therebetween)is collected on the sub-diffraction light detector 620A as a spot 682B3.Negative first order diffraction-light diffracted by the strip-shapedareas 650B31 and 650B32 is collected on the sub-diffraction lightdetector 630B2 while being also on the sub-diffraction light detector630B3 as a spot 683B3.

[0177] Positive first order diffraction light diffracted by the otherstrip-shaped areas 650F31, 650F32 and 650F33 is collected on thesub-diffraction light detector 620A as a spot 682F3. Negative firstorder diffraction light diffracted by the strip-shaped areas 650F31,650F32 and 650F33 is collected on the sub-diffraction light detector630B3 while being also on the sub-diffraction light detector 630B2 as aspot 683F3.

[0178] Positive first order diffraction light diffracted by thestrip-shaped areas 650B41, 650B42 and 650B43 of the fourth area 650 d(FIG. 6A; not adjacent to each other but interposing the areas 650F41and 650F42 therebetween) is collected on the sub-diffraction lightdetector 620B as a spot 682B4. Negative first order diffraction lightdiffracted by the strip-shaped areas 650341, 650B42 and 650B43 iscollected on the sub-diffraction light detector 630B1 while being alsoon the sub-diffraction light detector 630B2 as a spot 683B4.

[0179] Positive first order diffraction light diffracted by the otherstrip-shaped areas 650F41 and 650F42 is collected on the sub-diffractionlight detector 620B as a spot 682F4. Negative first order diffractionlight diffracted by the strip-shaped areas 650F41 and 650F42 iscollected on the sub-diffraction light detector 630B2 while being alsoon the sub-diffraction light detector 630B1 as a spot 683F4.

[0180] The light transmitted through the polarizing holographic face 650(0th order light) is collected substantially at an intersection of theseparation lines 611 and 612 of the transmission light detector 610 (ina central area of the transmission light detector 610) as a spot 681.The spot 681 is focused after the detection face of the transmissionlight detector 610.

[0181] The sub-diffraction light detectors 620A and 620B of the firstdiffraction light detector 620 each detect a light amount. A secondtracking error signal 6438 (TE2 signal) is obtained by subjecting thedetected light amounts to a subtraction performed by a subtracter 643 Areproduction signal 644 is obtained by subjecting the detected lightamounts to addition performed by an adder 644. The TE2 signalcorresponds to the TE2 signal detected by the photodetector 1190 shownin FIG. 1C.

[0182] The TE2 signal corresponds to a difference between the lightamount of the positive first order diffraction light diffracted by thefirst area 650 a and the fourth area 650 d of the polarizing holographicface 650 and the light amount of the positive first order diffractionlight diffracted by the second area 650 b and the third area 650 c ofthe polarizing holographic face 650. The reproduction signal correspondsto a sum of the light amount of the positive first order diffractionlight diffracted by the first area 650 a, the second area 650 b, thethird area 650 c and the fourth area 650 d.

[0183] Based on detection results of the sub-transmission lightdetectors 610A1, 610A2, 610B1 and 610B2, a calculator 641 of thephotodetector 600 outputs 610A1+610A2−610B1−610B2. The output from thecalculator 641 is a first tracking error signal 641 s (TE1 signal). TheTE1 signal corresponds to the TE1 signal detected by the photodetector1050 shown in FIG. 10B. Also based on detection results of thesub-transmission light detectors 610A1, 610A2, 610B1 and 610B2 acalculator 642 of the photodetector 600 outputs 610A1+610B2−610A2−610B1.The output from the calculator 642 is a third tracking error signal 642c (TE3 signal).

[0184] A calculator 645 outputs 630B1+630B3+630A2−630A1−630A3−630B2. Theoutput of the calculator 645 it a focusing error signal (FE signal).

[0185] In this example also, three types of tracking error signals (TE1,TE2 and TE3 signals) are obtained. Like in the first example, thesetracking error signals can be used in accordance with the type of theoptical disc. For example, in the case of an optical disc having a pitdepth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc),the control device 185 can use a TE3 signal as a tracking error signalwith respect to a pit signal (emboss signal).

[0186] In the case of an optical disc having a guide groove such as, forexample, a DVD-RAM disc or DVD-R disc, the control device 185 can be acalculation result value of TE2−k×TE1, obtained by using an appropriateconstant k, as a tracking error signal. In this case, the control device185 can update the value of k in accordance with the type of the opticaldisc.

[0187] Like in the first example, the degree of asymmetry of thetracking error signal caused by the shifting of the central axis of theobjective lens 160 with respect to the optical axis of the optical discapparatus can be sufficiently suppressed. Off-track while the trackingcontrol is performed can be solved. In this example, the polarizingholographic face 650 is divided into small strip-shaped areas. Usingthese small strip-shaped areas, a light component to be collected beforethe photodetector 600 and a light component to be collected after thephotodetector 600 are generated. The resultant diffraction light isdetected as an FE signal. Therefore the adverse influence of dust andstains present on the substrate 172 of the optical disc 170 is negated.Thus, the focusing error control is highly stable. In the fourthexample, unlike in the third example, the separation lines forseparating the sub-diffraction light detectors 630A1, 630A2 and 630A3and the separation lines for separating the sub-diffraction lightdetectors 630B1, 630B2 and 630B3 are along the diffraction direction ofthe light. Therefore, when there is a wavelength error or wavelengthshift, the spots on the second diffraction light detector 630 move alongthese separation lines. Thus, a detection error of focusing on theoptical disc can be sufficiently avoided.

[0188] The first and third examples have advantages that there is ampleroom for rotation adjustment of the photodetector, despite thepossibility of an FE detection error due to a wavelength error orwavelength shift. The separation lines between the sub-diffraction lightdetectors used for detecting an FE signal may or may not be along thediffraction direction of the light in accordance with the design idea.In the first, second, third and the following examples, the separationlines are perpendicular to the diffraction direction. The structures inthese examples can be modified so that the separation lines are parallelto the diffraction direction.

EXAMPLE 5

[0189]FIG. 7A shows a structure of a polarizing holographic face 750 ofan optical disc apparatus according to a fifth example of the presentinvention. FIG. 7B shows a structure of a photodetector 700 of theoptical disc apparatus according to the fifth example of the presentinvention. The optical disc apparatus according to the 5 fifth examplehas the same structure as that of the optical disc apparatus 100 in thefirst example except for the polarizing holographic face 750 and thephotodetector 700. The other elements will be described using thecorresponding reference numerals in FIG. 1A.

[0190] In FIG. 7A, the polarizing holographic face 750 is divided into afirst area 750 a, a second area 750 b, a third area 750 a and a fourtharea 750 d having different holographic patterns, along separation lines752 and 753. The separation line 752 is parallel to the rotationdirection of the optical disc 170, and the separation line 753 isperpendicular to the separation line 752. A light beam 751 reflected bythe optical disc 170 is substantially equally divided into four alongthe separation lines 752 and 753. The first area 750 a is furtherdivided into strip-shaped areas 750F11, 750B11, 750F12, 750B12 and750F13 along separation lines parallel to the separation line 753. Thesecond area 750 b is further divided into strip-shaped areas 750B21,750F21, 750B22, 750F22 and 750823 along separation lines parallel to theseparation line 753. The third area 750 c is further divided intostrip-shaped areas 750F31, 750B31, 750F32, 750B32 and 750F33 alongseparation lines parallel to the separation line 753. The fourth area750 d is further divided into strip-shaped areas 750B41, 750F41, 750B42.750F42 and 750B43 along separation lines parallel to the separation line753.

[0191] Negative first order diffraction light passing through thestrip-shaped areas having the letter “F” in their reference numerals(e.g., 750F11 or 750F22) is collected before the photodetector 700.Negative first order diffraction light passing through the strip-shapedareas having the letter “B” in their reference numerals (e.g., 750B11 or750B22) is collected after the photodetector 700.

[0192] Referring to FIG. 7B, the photodetector 700 includes atransmission light detector 710, a first diffraction light detector 720and a second diffraction light detector 730. The transmission lightdetector 710 is provided in a central area of the photodetector 700. Thefirst diffraction light detector 720 and the second diffraction lightdetector 730 are provided in a first outer area and a second outer area,respectively of the photodetector 700 so as to interpose thetransmission light detector 710 therebetween.

[0193] The transmission light detector 710 includes two sub-transmissionlight detectors 710A and 710B. The transmission light detector 710includes two areas 710C1 and 710C2. The sub-transmission light detector710A is provided in the area 710C1. The sub-transmission light detector710B is provided in the area 710C2. The areas 710C1 and 710C2 areseparated from each other by a separation line 711. The separation line711 extends parallel to the rotation direction of the optical disc 170.

[0194] The first diffraction light detector 720 provided in the firstouter area includes four sub-diffraction light detectors 720A1, 720A2,720B1 and 720B2. The first diffraction light detector 720 includes areas720C1, 720C2, 720C3 and 720C4. The sub-diffraction light detector 720A1is provided in the area 720C1. The sub-diffraction light detector 720A2is provided in the area 720C2. The sub-diffraction light detector 720B1in provided in the area 720C3. The sub-diffraction light detector 720B2is provided in the area 720C4.

[0195] The second diffraction light detector 730 provided in the secondouter area includes six sub-diffraction light detectors 730A1, 730A2,730A3, 730B3, 730B2 and 73033 like in the third example. Thesub-diffraction light detectors 730A1, 730B2 and 730A3 are electricallyconductive to each other The sub-diffraction light detectors 730B1,730A2 and 73033 are also electrically conductive to each other. Thesecond diffraction light detector 730 includes areas 730C1, 730C2,73OC3, 730C4, 730C5 and 730C6. The sub-diffraction light detector 730A1is provided in the area 730C1. The sub-diffraction light detector 730A2is provided in the area 730C2. The sub-diffraction light detector 730A3is provided in the area 730C3. The sub-diffraction light detector 730B1is provided in the area 730C4. The sub-diffraction light detector 730B2is provided in the area 730C5. The sub-diffraction light detector 730B3is provided in the area 730C6.

[0196] Positive first order diffraction light diffracted by thestrip-shaped areas 750811 and 750B12 of the first area 750 a of thepolarizing holographic face 750 (FIG. 7A; not adjacent to each other butinterposing the area 750F2 therebetween) is collected on thesub-diffraction light detector 720B1 as a spot 782B1. Negative firstorder diffraction light diffracted by the strip-shaped areas 750B11 and750B12 is collected on the sub-diffraction light detector 730B3 whilebeing also on the sub-diffraction light detector 73032 as a spot 783B1.

[0197] Positive first order diffraction light diffracted by the otherstrip-shaped areas 750F11, 750F12 and 750F13 is collected on thesub-diffraction light detector 720B1 as a spot 782F1. Negative firstorder diffraction light diffracted by the strip-shaped areas 750F11,750F12 and 750F13 is collected on the sub-diffraction light detector730B2 while being also on the sub-diffraction light detector 730B3 as aspot 783F1.

[0198] Positive first order diffraction light diffracted by thestrip-shaped areas 750B21, 750B22 and 7503B23 of the second area 750 b(FIG. 7A; not adjacent to each other but interposing the areas 750F21and 750F22 therebetween) is collected on the sub-diffraction lightdetector 720A2 as a spot 782B2. Negative first order diffraction lightdiffracted by the strip-shaped areas 750B21, 750B22 and 750B23 iscollected on the sub-diffraction light detector 730A2 while being alsoon the sub-diffraction light detector 730A1 as a spot 783B2.

[0199] Positive first order diffraction light diffracted by the otherstrip-shaped areas 750F21 and 750F22 is collected on the sub-diffractionlight detector 720A2 as a spot 782F2. Negative first order diffractionlight diffracted by the strip-shaped areas 750F21 and 750F22 iscollected on the sub-diffraction light detector 730A1 while being alsoon the sub-diffraction light detector 730A2 as a spot 783F2.

[0200] Positive first order diffraction light diffracted by thestrip-shaped areas 750B31 and 750B32 of the third area 750 c (FIG. 7A:not adjacent to each other but interposing the area 750F32 therebetween)is collected on the sub-diffraction light detector 720A1 as a spot782B3. Negative first order diffraction light diffracted by thestrip-shaped areas 750B31 and 750B32 is collected on the sub-diffractionlight detector 730A2 while being also on the sub-diffraction lightdetector 730A3 as a spot 783B3.

[0201] Positive first order diffraction light diffracted by the otherstrip-shaped areas 750F31, 750F32 and 750F33 is collected on thesub-diffraction light detector 720A1 as a spot 782F3. Negative firstorder diffraction light diffracted by the strip-shaped areas 750F31,750F32 and 750F33 is collected on the sub-diffraction light detector730A3 while being also on the sub-diffraction light detector 730A2 as aspot 783F3.

[0202] Positive first order diffraction light diffracted by thestrip-shaped areas 750B41, 750B42 and 750B43 of the fourth area 750 d(FIG. 7A: not adjacent to each other but interposing the areas 750F41and 750F42 therebetween) is collected on the sub-diffraction lightdetector 720B2 as a spot 782B4. Negative first order diffraction lightdiffracted by the strip-shaped areas 750B41. 750B42 and 750B43 iscollected on the sub-diffraction light detector 730B1 while being alsoon the sub-diffraction light detector 73092 as a spot 783B4.

[0203] Positive first order diffraction light diffracted by the otherstrip-shaped areas 750P41 and 750F42 is collected on the sub-diffractionlight detector 720B2 as a spot 782P4. Negative first order diffractionlight diffracted by the strip-shaped areas 750F41 and 750F42 iscollected on the sub-diffraction light detector 730B2 while being alsoon the sub-diffraction light detector 730B1 as a spot 783F4.

[0204] The light transmitted through the polarizing holographic face 750(0th order light) is collected at a substantial center of the separationline 711 as a spot 781. The spot 781 is focused before the detectionface of the transmission light detector 710. The sub-transmission lightdetectors 710A and 710B of the transmission light detector 710 eachdetect a light amount. A tracking error signal 7418 (TE1 signal) isobtained by subjecting the detected light amounts to a subtractionperformed by a subtracter 741. A reproduction signal 7428 is obtained bysubjecting the detected light amounts to addition performed by an adder742. The TE1 signal corresponds to the TE1 signal detected by thephotodetector 1050 shown in FIG. 10B.

[0205] In this example also, the transmission light detector 710, whichis substantially rectangular, is divided into sub-transmission lightdetectors 710A and 710B, which are also substantially rectangular. Inthis case, the difference between the light amounts detected by thesub-transmission light detectors 710A and 710B separated from each otherby the separation line 711 which extends parallel to the rotationdirection of the optical disc 170 is the TE1 signal. The sum of thelight amounts detected by the sub-transmission light detectors 710A and710B is the reproduction signal.

[0206] Based on detection results of the sub-diffraction light detectors720A1, 720A2, 720B1 and 720B2, a calculator 743 of the photodetector 700outputs 720A1+720A2−720B1−720B2. The output from the calculator 743 is asecond tracking error signal 743 a (TE2 signal). The TE2 signalcorresponds to the TE2 signal detected by the photodetector 1190 shownin FIG. 1C. Also based on detection results of the sub-diffraction lightdetectors 720A1, 720A2, 720B1 and 720B2, a calculator 745 of thephotodetector 700 outputs 720A1+720B2−720A2−720B1. The output from thecalculator 744 is a third tracking error signal 7435 (TE3 signal).

[0207] Based on detection results of the sub-diffraction light detectors730A1, 730A2, 730A3, 730B1, 730B2 and 730B3, a calculator 745 outputs730B1+730B3+730A2−730A1−730A3−730B2. The output of the calculator 745 isa focusing error signal 745 s (FE signal).

[0208] Like in the first example, the phase distribution of the wavesurface of the light immediately after being transmitted through thepolarizing holographic face 750 has a sawtooth-like or step-like shape.The phase distribution 19, or the holographic pattern, has asawtooth-like or step-like shape, the pattern being continuous oversequential cycles. In this example, the phase difference between thefirst step and the second step, and the phase difference between thesecond step and the third step are significantly small. In this manner,the diffraction light amount ratio can be 70% for the 0th order light,15% for the positive first order diffraction light and 5% for thenegative first order diffraction light. Since the diffraction efficiencyof the 1st order diffraction light is small, the diffraction loss isalso small. As a result, the total diffraction light amount (i.e.,70+15+5=90%) is larger than that of the first example. Thus, the lightamounts can be adjusted so as to be largest for the transmission light,second largest for the positive first order diffraction light, andsmallest for the negative first order diffraction light.

[0209] In this example also three types of tracking error signals (TE1,TE2 and TE3 signals) are obtained. Like in the first example, thesetracking error signals can be used in accordance with the type of theoptical disc. For example, in the case of an optical disc having a pitdepth corresponding to about ¼ of the wavelength (e. g., DVD-ROM disc),the control device 185 can use a TE3 signal as a tracking error signalwith respect to a pit signal (emboss signal).

[0210] In the case of an optical disc having a guide groove such as, forexample, a DVD-RAM disc or DVD-R disc, the control device 185 can use acalculation result value of TE2−k×TE1, obtained by using an appropriateconstant k, as a tracking error signal. In this case, the control device185 can update the value of k in accordance with the type of the opticaldisc.

[0211] Like in the first example, the degree of asymmetry of thetracking error signal caused by the shifting of the central axis of theobjective lens 160 with respect to the optical axis of the optical discapparatus 300 can be sufficiently suppressed. Off-track while thetracking control is performed can be solved. In this example, thepolarizing holographic face 750 is divided into small strip-shapedareas. Using these small strip-shaped areas, a light component to becollected before the photodetector 700 and a light component to becollected after the photodetector 700 are generated. The resultantdiffraction light is detected as an FE signal. Therefore, the adverseinfluence of dust and stains present on the substrate 172 of the opticaldisc 170 is negated. Thus, the focusing error control is highly stable.

[0212] In the fifth example, the detected light amount of the 0th orderlight (transmission light) is used to detect a reproduction signal. Thedetection index=70/{square root}{square root over (2)}=about 50. Ahigher S/N ratio than that of the first example is guaranteed.

EXAMPLE 6

[0213]FIG. 8A shows a structure of a polarizing holographic face 850 ofan optical disc apparatus according to a sixth example of the presentinvention. FIG. 8B shows a structure of a photodetector 800 of theoptical disc apparatus according to the sixth example of the presentinvention. The optical disc apparatus according to the sixth example hasthe same structure as that of the optical disc apparatus 100 in thefirst example except for the polarizing holographic face 850 and thephotodetector 800. The other elements will be described using thecorresponding reference numerals in FIG. 1A.

[0214] In FIG. 8A, the polarizing holographic face 850 is divided into afirst area 850 a, a second area 850 b, a third area 850 a and a fourtharea 850 d having different holographic patterns, along separation lines852 and 853. The separation line 852 is parallel to the rotationdirection of the optical disc 170, and the separation line 853 isperpendicular to the separation line 852. A light beam 851. reflected bythe optical disc 170 is substantially equally divided into four alongthe separation lines 852 and 853. The first area 850 a is furtherdivided into strip-shaped areas 850F11, 850B11, 850F12, 850B12 and850F13 along separation lines parallel to the separation line 853. Thesecond area 850 b is further divided into strip-shaped areas 850B21,850F21, 850B22, 850F22 and 850B23 along separation lines parallel to theseparation line 853. The third area 850 c is further divided intostrip-shaped areas 85031, 850B31, 850F32, 850B32 and 850F33 alongseparation lines parallel to the separation line 853. The fourth area850 d is further divided into strip-shaped areas 850B41, 850F41, 850B42,850F42 and 850B43 along separation lines parallel to the separation line853.

[0215] Negative first order diffraction light passing through thestrip-shaped areas having the letter “F” in their reference numerals(e.g., 850F11 or 850F22) is collected before the photodetector 800.Negative first order diffraction light passing through the strip-shapedareas having the letter “B” in their reference numerals (e.g., 850B11 or850B22) is collected after the photodetector 800.

[0216] Referring to FIG. 8B, the photodetector 800 includes atransmission light detector 810, a first diffraction light detector 820and a second diffraction light detector 830. The transmission lightdetector 810 is provided in a central area of the photodetector 800. Thefirst diffraction light detector 820 and the second diffraction lightdetector 830 are provided in a first outer area and a second outer area,respectively, of the photodetector 800 so as to interpose thetransmission light detector 810 therebetween.

[0217] The transmission light detector 810 includes two sub-transmissionlight detectors 810A and 810B. The transmission light detector 810includes two areas 810C1 and 810C2. The sub-transmission light detector810A is provided in the area 810C1. The sub-transmission light detector810B provided in the area 810C2. The areas 810C1 and 810C2 are separatedfrom each other by a separation line 811. The separation line 811extends parallel to the rotation direction of the optical disc 170.

[0218] The first diffraction light detector 820 provided in the firstouter area includes two sub-diffraction light detectors 820A and 820B.The first diffraction light detector 820 includes areas 820C1 and 820C2.The sub-diffraction light detector 820A is provided in the area 820C1.The sub-diffraction light detector 820B is provided in the area 820C2.

[0219] The second diffraction light detector 830 provided in the secondouter area includes six sub-diffraction light detectors 830A1, 830A2,830A3, 830B1, 830B2 and 830B3 like in the third example. Thesub-diffraction light detectors 830A1, 830B2 and 830A3 are electricallyconductive to each other. The sub-diffraction light detectors 83OB1,830A2 and 830B3 are also electrically conductive to each other. Thesecond diffraction light detector 830 includes areas 830C1, 830C2,830C3, 830C4, 830C5 and 830C6. The sub-diffraction light detector 830A1is provided in the area 830C1. The sub-diffraction light detector 830A2is provided in the area 830C2. The sub-diffraction light detector 830A3is provided in the area 830C3. The sub-diffraction light detector 830B1is provided in the area 830C4. The sub-diffraction light detector 830B2is provided in the area 830C5. The sub-diffraction light detector 830B3is provided in the area 830C6.

[0220] Positive first order diffraction light diffracted by thestrip-shaped areas 850B11 and 850B12 of the first area 850 a of thepolarizing holographic face 850 (FIG. 8A; not adjacent to each other butinterposing the area 850F12 therebetween) is collected on thesub-diffraction light detector 8201 as a spot 8823. Negative first orderdiffraction light diffracted by the strip-shaped areas 850B11 and 850B12is collected on the sub-diffraction light detector 830B3 while beingalso on the sub-diffraction light detector 830B2 as a spot 883B1.

[0221] Positive first order diffraction light diffracted by the otherstrip-shaped areas 850F11, 850F12 and 850F13 is collected on thesub-diffraction light detector 820B as a spot 382F1. Negative firstorder diffraction light diffracted by the strip-shaped areas 850F11,850F12 and 850F13 is collected on the sub-diffraction light detector830B2 while being also on the sub-diffraction light detector 830B3 as aspot 883F1.

[0222] Positive first order diffraction light diffracted by thestrip-shaped areas 850B21, 850B22 and 850B23 of the second area 850 b(FIG. 8A; not adjacent to each other but interposing the areas 850F21and 850F22 therebetween) is collected on the sub-diffraction lightdetector 820A as a spot 882B2. Negative first order diffraction lightdiffracted by the strip-shaped areas 850B21, 850B22 and 850B23 iscollected on the sub-diffraction light detector 830A2 while being alsoon the sub-diffract ion light detector 830A1 as a spot 883B2.

[0223] Positive first order diffraction light diffracted by the otherstrip-shaped areas 850F21 and 850F22 is collected on the sub-diffractionlight detector 820A as a spot 882F2. Negative first order diffractionlight diffracted by the strip-shaped areas 850F21 and 850F22 iscollected on the sub-diffraction light detector 830A1 while being alsoon the sub-diffraction light detector 830A2 as a spot 883P2.

[0224] Positive first order diffraction light diffracted by thestrip-shaped areas 850B31 and 850B32 of the third area 850 a (FIG. 8A:not adjacent to each other but interposing the area 850F32 therebetween)is collected on the sub-diffraction light detector 820A as a spot 882B3.Negative first order diffraction light diffracted by the strip-shapedareas 850B31 and 850B32 is collected on the sub-diffraction lightdetector 830A2 while being also on the sub-diffraction light detector830A3 as a spot 88333.

[0225] Positive first order diffraction light diffracted by the otherstrip-shaped areas 850F31, 83OP32 and 850F33 is collected on thesub-diffraction light detector 820A as a spot 882F3. Negative firstorder diffraction light diffracted by the strip-shaped areas 850F31,850P32 and 850F33 is collected on the sub-diffraction light detector830A3 while being also on the sub-diffraction light detector 830A2 as aspot 883F3.

[0226] Positive first order diffraction light diffracted by thestrip-shaped areas 850B41, 850B42 and 850B43 of the fourth area 850 d(FIG. 8A; not adjacent to each other but interposing the areas 850F41and 850F42 therebetween) is collected on the sub-diffraction lightdetector 820B as a spot 882B4. Negative first order diffraction lightdiffracted by the strip-shaped areas 850B41, 850B42 and 8S0B43 iscollected on the sub-diffraction light detector 830B1 while being alsoon the sub-diffraction light detector 830D2 as a spot 88334.

[0227] Positive first order diffraction light diffracted by the otherstrip-shaped areas 850F41 and 850F42 is collected on the sub-diffractionlight detector 820B as a spot 882F4. Negative first order diffractionlight diffracted by the strip-shaped areas 850F41 and 850F42 iscollected on the sub-diffraction light detector 830B2 while being alsoon the sub-diffraction light detector 830B1 as a spot 883F4.

[0228] The light transmitted through the polarizing holographic face 850(0th order light) is collected at a substantial center of the separationline 811 as a spot 881 The sub-transtission light detectors 810A and810B of the transmission light detector 810 each detect a light amount.A first tracking error signal 841 s (TE1 signal) is obtained bysubjecting the detected light amounts to a subtraction performed by asubtracter 841. A reproduction signal 842 s is obtained by subjectingthe detected light amounts to addition performed by an adder 842. TheTE1 signal corresponds to the TE1 signal detected by the photodetector1050 shown in FIG. 10B.

[0229] The sub-diffraction light detectors 820A and 820B of the firstdiffraction light detector 820 each detect a light amount. A secondtracking error signal 842 s (TE2 signal) is obtained by subjecting thedetected light amounts to a subtraction performed by subtracter 843. TheTE2 signal corresponds to the TE2 signal detected by the photodetector1190 shown in FIG. 11C.

[0230] Based on detection results of the sub-diffraction light detectors830A1, 830A2, 830A3, 830B1, 830B2 and 830B3, a calculator 845 outputs830B1+830B3+830A2−830A1−830A3−830B2. The output of the calculator 845 isa focusing error signal 845 s (FE signal).

[0231] Unlike in the first example, the phase distribution of the wavesurface of the light immediately after being transmitted through thepolarizing holographic face 850 has a cyclic rectangular shape(so-called two-level grating shape). The phase difference between alower step and an upper step is significantly small. Therefore, thediffraction light amount ratio can be 70% for the 0th order light, 10%for the positive first order diffraction light and 10% for the negativefirst order diffraction light. Since the diffraction efficiency of the±1st order diffraction light is small, the diffraction loss is alsosmall. As a result, the total diffraction light amount (i.e.,70+10+10=90%) is larger than that of the first example. Thus, the lightamounts can be adjusted so as to be larger for the transmission lightand smaller for the positive first order diffraction light or thenegative first order diffraction light. The light amounts can beadjusted so as to be largest for the transmission light, second largestfor the negative first order diffraction light, and smallest for thepositive first order diffraction light.

[0232] In this example, two types of tracking error signals (TE1 and TE2signals) are obtained. Accordingly, like in the first example, thecontrol device 185 can use a calculation result value of TE2−k×TE1obtained by using an appropriate constant k, as a tracking error signalin this case, the control device 185 can update the value of k inaccordance with the type of the optical disc.

[0233] Like in the first example, the degree of asymmetry of thetracking error signal caused by the shifting of the central axis of theobjective lens 160 with respect to the optical axis of the optical discapparatus 300 can be sufficiently suppressed. Off-track while thetracking control is performed can be solved. In this example, thepolarizing holographic face 850 is divided into small strip-shapedareas. Using these small strip-shaped areas, a light component to becollected before the photodetector 800 and a light component to becollected after the photo detector 800 are generated. The resultantdiffraction light is detected as an FE signal. Therefore, the adverseinfluence of dust and stains present on the substrate 172 of the opticaldisc 170 is negated. Thus, the focusing error control is highly stable.

[0234] In the sixth example, the detected light amount of the 0th orderlight is used to detect a reproduction signal. The detectionindex=70/{square root}{square root over (2)}=about 50. A higher S/Nratio 25 than that of the first example is guaranteed. Since the thirdtracking error signal (TE3 signal) is not obtained, the control device185 cannot perform tracking of the pit signal (emboss signal) of theoptical disc 170 having a pit depth corresponding to about ¼ of thewavelength, such as, for example, a DVD-ROM disc.

[0235] In the sixth example, the 0th order light is used to detect areproduction signal. Alternatively, the detected light amount of thepositive first order diffraction light can be used. The light amountsdetected by the sub-diffraction light detectors 820A and 820B can beadded by the adder 844 to obtain the reproduction signal 844 s. In thiscase, the phase differential distribution of the wave surface of lightimmediately after being transmitted through the polarizing holographicface 850 is 20% for the 0th order light, 47.6% for the positive firstorder diffraction light, and 12.4% for the negative first orderdiffraction light. The detection index of the reproduction signal is47.6/{square root}{square root over (2)}=34.

EXAMPLE 7

[0236]FIG. 9A shows a structure of a polarizing holographic face 950 ofan optical disc apparatus according to a seventh example of the presentinvention. FIG. 9B shows a structure of a photodetector 900 of theoptical disc apparatus according to the seventh example of the presentinvention. The optical disc apparatus according to the seventh examplehas the same structure as that of the optical disc apparatus 100 in thefirst example except for the polarizing holographic face 950 and thephotodetector 900. The other elements will be described using thecorresponding reference numerals in FIG. 1A.

[0237] In FIG. 9A, the polarizing holographic face 950 is divided into afirst area 950 a, a second area 950 b, a third area 950 a and a fourtharea 950 d having different holographic patterns, along separation lines952 and 953. The separation line 952 is parallel to the rotationdirection of the optical disc 170, and the separation line 953 isperpendicular to the separation line 952. A light beam 951 reflected bythe optical disc 170 is substantially equally divided into four alongthe separation lines 952 and 953.

[0238] The first area 950 a is further divided into strip-shaped areas950F11, 950B11, 950F12, 950B12 and 950F13 along separation linesparallel to the separation line 953. The second area 950 b is furtherdivided into strip-shaped areas 950B21, 950F21, 950B22, 950F22 and950B23 along separation lines parallel to the separation line 953. Thethird area 950 c is further divided into strip-shaped areas 950F31,950B31, 950F32, 950B32 and 950F33 along separation lines parallel to theseparation line 953. The fourth area 9504 is further divided intostrip-shaped areas 950B41, 950F41, 950B42, 950742 and 950B43 alongseparation lines parallel to the separation line 953.

[0239] Negative first order diffraction light passing through thestrip-shaped areas having the letter “F” in their reference numerals(e.g., 950F11 or 950F22) is collected before the photodetector 900.Negative first order diffraction light passing through the strip-shapedareas having the letter “B” in their reference numerals (e.g., 950B11 or950B22) is collected after the photodetector 900.

[0240] Referring to FIG. 9B, the photodetector 900 includes atransmission light detector 910, a first diffraction light detector 920and a second diffraction light detector 930. The transmission lightdetector 910 is provided in a central area of the photodetector 900. Thefirst diffraction light detector 920 and the second diffraction lightdetector 930 are provided in a first outer area and a second outer area,respectively, of the photodetector 900 so as to interpose thetransmission light detector 910 therebetween.

[0241] The transmission light detector 910 includes foursub-transmission light detectors 910A1, 910A2, 910B1 and 910B2. Thetransmission light detector 910 includes four areas 910C1, 910C2, 910C3and 910C4. The sub-transmission light detector 910A1 is provided in thearea 910C1. The sub-transtission light detector 910A2 is provided in thearea 910C2. The sub-transmission light detector 910B1 is provided in thearea 910C3. The sub-transmission light detector 910B2 is provided in thearea 910C4. The areas 910C1, 910C2, 910C3 and 910C4 are separated fromeach other by separation lines 911 and 912 which are perpendicular toeach other. The separation line 911 extends parallel to the rotationdirection of the optical disc 170.

[0242] The first diffraction light detector 920 has an area 920C. Thefirst diffraction light detector 920 is provided in the area 920C.

[0243] The second diffraction light detector 930 provided in the secondouter area includes six sub-diffraction light detectors 930A1, 930A2,930A3, 930B1, 930B2 and 930B3 like in the third example. Thesub-diffraction light detectors 930A1 and 930A3 are electricallyconductive to each other. The sub-diffraction light detectors 93081 and930B3 are also electrically conductive to each other. The seconddiffraction light detector 930 includes areas 930C1, 930C2, 930C3,930C4, 930CS and 930C6. The sub-diffraction light detector 930A1 isprovided in the area 930C1. The sub-diffraction light detector 930A2 isprovided in the area 930C2. The sub-diffraction light detector 930A3 isprovided in the area 930C3. The sub-diffraction light detector 93081 isprovided in the area 930C4. The sub-diffraction light detector 930B2 isprovided in the area 930C5. The sub-diffraction light detector 930B3 isprovided in the area 930C6.

[0244] Positive first order diffraction light diffracted by thestrip-shaped areas 950B11 and 950B12 of the first area 950 a of thepolarizing holographic face 950 (FIG. 9A; not adjacent to each other butinterposing the area 950F12 therebetween) is collected on the firstdiffraction light detector 920 as a spot 982B1. Negative first orderdiffraction light diffracted by the strip-shaped areas 950B13 and 950B12is collected on the sub-diffraction light detector 930B3 while beingalso on the sub-diffraction light detector 930B2 as a spot 983B1.

[0245] Positive first order diffraction light diffracted by the otherstrip-shaped areas 950F11, 950P12 and 950P13 is collected on the firstdiffraction light detector 920 as a spot 982F1. Negative first orderdiffraction light diffracted by the strip-shaped areas 950F11, 950F12and 950F13 lo collected on the sub-diffraction light detector 933B2while being also on the sub-diffraction light detector 930B3 as a spot983F1.

[0246] Positive first order diffraction light diffracted by thestrip-shaped areas 950B21, 950B22 and 950B23 of the second area 950 b(FIG. 9A; not adjacent to each other but interposing the areas 950F21and 950F22 therebetween) is collected on the first diffraction lightdetector 920 as a spot 982B2. Negative first order diffraction lightdiffracted by the strip-shaped areas 950B21, 950B22 and 950B23 iscollected on the sub-diffraction light detector 930A2 while being alsoon the sub-diffraction light detector 930A1 as a spot 983B2.

[0247] Positive first order diffraction light diffracted by the otherstrip-shaped areas 950F21 and 950F22 is collected on the firstdiffraction light detector 920 as a spot 982F2. Negative first orderdiffraction light diffracted by the strip-shaped areas 950F21 and 950F22is collected on the sub-diffraction light detector 930A1 while beingalso on the sub-diffraction light detector 930A2 as a spot 983F2.

[0248] Positive first order diffraction light diffracted by thestrip-shaped areas 950B31 and 950832 of the third area 950 a (FIG. 9A;not adjacent to each other but interposing the area 950F32 therebetween)is collected on the first diffraction light detector 920 as a spot982B3. Negative first order diffraction light diffracted by thestrip-shaped areas 950B31 and 950B32 is collected on the sub-diffractionlight detector 930A2 while being also on the sub-diffraction lightdetector 930A3 as a spot 983B3 Positive first order diffraction lightdiffracted by the other strip-shaped areas 950F31, 950F32 and 950F33 iscollected on the first diffraction light detector 920A as a spot 982F3.Negative first order diffraction light diffracted by the strip-shapedareas 950F31, 950F32 and 950F33 is collected on the sub-diffractionlight detector 930A3 while being also on the sub-diffraction lightdetector 930A2 as a spot 983F3.

[0249] Positive first order diffraction light diffracted by thestrip-shaped areas 950B41, 950B42 and 950B43 of the fourth area 950 d(FIG. 9A; not adjacent to each other but interposing the areas 950F41and 950F42 therebetween) is collected on the first diffraction lightdetector 920 as a spot 98284. Negative first order diffraction lightdiffracted by the strip-shaped areas 950B41, 940B42 and 950B43 iscollected on the sub-diffraction light detector 930B1 while being alsoon the sub-diffraction light detector 930B2 as a spot 983B4.

[0250] Positive first order diffraction light diffracted by the otherstrip-shaped areas 950F41 and 950P42 is collected on the firstdiffraction light detector 920 as a spot 982F49 Negative first orderdiffraction light diffracted by the strip-shaped areas 950F41 and 950P42is collected on the sub-diffraction light detector 930B2 while beingalso on the sub-diffraction light detector 930B1 as a spot 983F4.

[0251] The light transmitted through the polarizing holographic face 950(0th order light) is collected substantially at an intersection of theseparation lines 911 and 912 (in a central area of the transmissionlight detector 910) a spot 981.

[0252] Based on the detection result of the first diffraction lightdetector 920, a reproduction signal lid is obtained.

[0253] Based on detection results of the sub-transmission lightdetectors 910A1, 910A2, 910B1 and 910B2, a calculator 941 of thephotodetector 900 outputs 910A1+910A2−910B1−910B2. The output from thecalculator 941 is a first tracking error signal 941 s (TE1 signal). TheTE1 signal corresponds to the TE1 signal detected by the photodetector1050 shown in FIG. 10B. Also based on detection results of thesub-transmission light detectors 910A1, 930A2, 910B1 and 910B2, acalculator 942 of the photodetector 900 outputs 910A1+910B2−910A2−910B1.The output from the calculator 942 is a third tracking error signal 9428(TE3 signal).

[0254] Based on detection results of the sub-diffraction light detectors930A1, 930A2, 930A3, 930B1, 930B2, and 930B3, a detection signal 11 ecorresponding to 930B1+930B3, a detection signal 11 f corresponding to93082, a detection signal 11 g corresponding to 930A1+930A3, and adetection signal 11 h corresponding to 930A2 are obtained. A secondtracking error signal (TE2 signal) is obtained by calculation of 11 g+11h−11 e−11 f. A focusing error signal (FE signal) is obtained bycalculation of 11 e−11 f−11 g+11 h. The TE2 signal corresponds to theTE2 signal detected by the photodetector 1190 shown in FIG. 11C.

[0255] In this example, the phase distribution of the wave surface ofthe light Immediately after being transmitted through the polarizingholographic face 950 is similar to that of the first example. The ratioof the diffracted light amount allocated for the 0th order light amount(transmission light amount) is 20%, the ratio for the positive firstorder diffraction light amount is 47.6%, and the ratio for the negativefirst order diffraction light amount is 12.4%.

[0256] In this example also, three types of tracking error signals (TE1,TE2 and TE3 signals) are obtained. Like in the first example, thesetracking error signals can be used in accordance with the type of theoptical disc. For example, in the case of an optical disc having a pitdepth corresponding to about ¼ of the wavelength (e.g., DVD-ROM disc),the control device 185 can use a TE3 signal as a tracking error signalwith respect to a pit signal (emboss signal).

[0257] In the case of an optical disc having a guide groove such as, forexample, a DVD-RAM disc or DVD-R disc, the control device 185 can use acalculation result value of TE2−k×TE1, obtained by using an appropriateconstant k, as a tracking error signal. In the case, the control device185 can update the value of k in accordance with the type of the opticaldisc.

[0258] Like in the first example, the degree of asymmetry of thetracking error signal caused by the shifting of the central axis of theobjective lens 160 with respect to the optical axis of the optical discapparatus 300 can be sufficiently suppressed. Off-track while thetracking control is performed can be solved. In this example, thepolarizing holographic face 950 is divided into small strip-shapedareas. Using these small strip-shaped areas, a light component to becollected before the photodetector 900 and a light component to becollected after the photodetector 900 are generated. The resultantdiffraction light is detected as an FE signal. Therefore, the adverseinfluence of dust and stains present on the substrate 172 of the opticaldisc 170 is negated Thus., the focusing error control is highly stable.In the seventh example, one detector (the first diffraction lightdetector 920) is used to detect a reproduction signal. The detectionindex is about 47.6. A higher S/N ratio than that of the first exampleis guaranteed.

[0259] According to the present invention, two types of tracking errorsignals (TE1 and TE2 signals), which are conventionally detected, can besimultaneously detected. Thus, the control device 185 generates asufficiently accurate tracking error signal from the two types oftracking error signals. The control device 185 can use a calculationresult value of TE2−k×TE1, obtained by using an appropriate constant k,as a tracking error signal. The polarizing holographic element and thephotodetector can be divided in other manners. The diffractionefficiency can be distributed in different manners. The holographicelement can be a non-polarizing holographic element or other lightdistribution element.

[0260] According to the present invention, using a calculation resultvalue of TE2−k×TE1 as a tracking error signal, the degree of asymmetryof the tracking error signal caused by the shifting of the objectivelens when the laser light crosses the pits is sufficiently suppressed.Off track while the tracking control is performed can be solved.Therefore, satisfactory and stable recording and reproduction can berealized. In the case where a light distribution section, such as apolarizing holographic element or the like, has a pattern havingsawtooth-like or step-like shape including three or more steps (thepattern being continuous over sequential cycles), the reproductionsignal can have a sufficiently high S/N ratio and thus a high signalreproduction performance is obtained.

[0261] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

What is claimed is:
 1. An optical disc apparatus capable of mounting anoptical disc, comprising: a light source for emitting light; anobjective lens for collecting the light emitted by the light source onthe optical disc; a first light distribution section integrally movablewith the objective lens, the first light distribution section includinga first area and a second area, the first light distribution sectionoutputting the light reflected by the optical disc and translatedthrough the first area or the second area as transmission light,outputting the light reflected by the optical disc and diffracted by thefirst area as first diffraction light, and outputting the lightreflected by the optical disc and diffracted by the second area assecond diffraction light; a transmission light detection section fordetecting the transmission light and outputting a TE1 signal indicatingan offset of the detected transmission light; a first diffraction lightdetection section for detecting the first diffraction light and thesecond diffraction light, and outputting a TE2 signal indicating adifference between a light amount of the detected first diffractionlight and a light amount of the detected second diffraction light; and acontrol device for generating a tracking error signal for the opticaldisc based on the TE1 signal and the TE2 signal.
 2. An optical discapparatus according to claim 1, further comprising a second lightdistribution section for directing the transmission light toward thetransmission light detection section and directing the first diffractionlight and the second diffraction light toward the first diffractionlight detection section.
 3. An optical disc apparatus according to claim1, wherein: the transmission light detection section includes a firstsub-transmission light detection section and a second sub-transmlssionlight detection section, first transmission light is defined as a partof the transmission light, which is detected by the firstsub-transmission light detection section, and second transmission lightis defined as a part of the transmission light, which is detected by thesecond sub-transmission light detection section, and the offset of thetransmission light is defined as a difference between a light amount ofthe first transmission light and a light amount of the secondtransmission light.
 4. An optical disc apparatus according to claim 1,wherein the first diffraction light detection section includes a firstsub-diffraction light detection section for detecting the firstdiffraction light and a second sub-diffraction light detection sectionfor detecting the second diffraction light.
 5. An optical disc apparatusaccording to claim 1, wherein the control device obtains the trackingerror signal by TE2−k×TE1.
 6. An optical disc apparatus according toclaim 3, wherein: the transmission light detection section includes athird area and a fourth area, the first sub-transmission light detectionsection is provided in the third area, and the second sub-transmissionlight detection section is provided in the fourth area, and a borderbetween the third area and the fourth area is parallel to a rotationdirection of the optical disc.
 7. An optical disc apparatus according toclaim 4, wherein: the first diffraction light detection section includesa fifth area and a sixth area, the first sub-diffraction light detectionsection is provided in the fifth area, and the second sub-diffractionlight detection section is provided in the sixth area, and a borderbetween the fifth area and the sixth area is parallel to a rotationdirection of the optical disc.
 8. An optical disc apparatus according toclaim 5, wherein the control device updates a value of k in accordancewith a logical product of a numerical aperture (NA) of the objectivelens and a pit pitch (P) of the optical disc in a diameter direction ofthe optical disc (NA×P).
 9. An optical disc apparatus according to claim5, wherein a value of k is 0.5×S2/S1 or less, wherein S1 is a lightamount of the transmission light detected by the transmission lightdetection section, and S2 is a light amount of the diffraction lightdetected by the first diffraction light detection section.
 10. Anoptical disc apparatus according to claim 8, wherein the control devicesets the value of k at zero when the logical product of the numericalaperture (NA) of the objective lens and the pit pitch (P) of the opticaldisc in the diameter direction of the optical disc (NA×P) is 0.9 timesor more of the wavelength of the light incident on the optical disk. 11.An optical disc apparatus according to claim 5, wherein the controldevice sets a value of k so that an average output level of TH2−k×TE1 issubstantially zero when the control device shifts the objective lens ina diameter direction of the optical disc without performing trackingcontrol.
 12. An optical disc apparatus according to claim 1, furthercomprising an aberration section for providing the transmission lightwith an aberration, wherein: the transmission light detection sectionincludes a third area, a fourth area, a seventh area and an eighth thefirst sub-transmission light detection section is provided in the thirdarea, the second sub-transmission light detection section is provided inthe fourth area, the third sub-transmlssion light detection section isprovided in the seventh area, the fourth sub-transmission lightdetection section is provided in the eighth area. a border between thethird area and the fourth area is parallel to a rotation direction ofthe optical disc, a border between the third area and the eighth area isparallel to a diameter direction of the optical disc, a border betweenthe fourth area and the seventh area is parallel to a diameter directionof the optical disc, a border between the seventh area and the eightharea is parallel to a rotation direction of the optical disc, the thirdarea is orthogonal with respect to the seventh area, the fourth area isorthogonal with respect to the eighth area, and the control deviceobtains a focusing error signal for the optical disc based on adifference between a sum of a light amount of the transmission lightprovided with the aberration and detected by the first sub-transmissionlight detection section and a light amount of the transmission lightprovided with the aberration and detected by the third sub-transmissionlight detection section, and a sum of a light amount of the transmissionlight provided with the aberration and detected by the secondsub-transmission light detection section and a light amount of thetransmission light provided with the aberration and detected by thefourth sub-transmlssion light detection section.
 13. An optical discapparatus according to claim 1, wherein: the first light distributionsection includes a ninth area and a tenth area, the first lightdistribution section outputs the light reflected by the optical disc anddiffracted by the ninth area of the first light distribution section asthird diffraction light, and outputs the -light reflected by the opticaldisc and diffracted by the tenth area of the first light distributionsection as fourth diffraction light, the first diffraction lightdetection section includes a first sub-diffraction light detectionsection, a second sub-diffraction light detection section, a thirdsub-diffraction light detection section, a fourth sub-diffraction lightdetection section, a fifth sub-diffraction light detection section, anda sixth sub-diffraction light detection section, the first diffractionlight is detected by the first sub-diffraction detection section and thesecond sub-diffraction detection section, the second diffraction lightis detected by the fifth sub-diffraction detection section and the sixthsub-diffraotion detection section, the third diffraction light isdetected by the fourth sub-diffraction detection section and the fifthsub-diffraction detection section, the fourth diffraction light isdetected by the second sub-diffraction detection section and the thirdsub-diffraction detection section, and the control device obtains afocusing error signal for the optical disc based on a difference betweena total light amount of the diffraction light detected by the firstsub-diffraction light detection section, the third sub-diffraction lightdetection section and the fifth sub-diffraction light detection section,and a total light amount of the diffraction light detected by the secondsub-diffraction light detection section, the fourth sub-diffractionlight detection section and the sixth sub-diffraction light detectionsection.
 14. An optical disc apparatus according to claim 1, furthercomprising a second diffraction light detection section, wherein: thefirst light distribution section outputs the light, reflected by theoptical disc and diffracted by the first area of the first lightdistribution section separately from the first diffraction light, asfifth diffraction light, and outputs the light, reflected by the opticaldisc and diffracted by the second area of the first light distributionsection separately from the second diffraction light, as sixthdiffraction light, the second diffraction light detection sectionincludes a seventh sub-diffraction light detection section and an eighthsub-diffraction light detection section, and the control device obtainsa focusing error signal for the optical disc based on a differencebetween a light amount of the fifth diffraction light detected by theseventh sub-diffraction light detection section and a light amount ofthe sixth sub-diffraction light detected by the eighth sub-diffractionlight detection section.
 15. An optical disc apparatus according toclaim 1, wherein: the first light distribution section includes aholographic element having a pattern having sawtooth-like or step-likeshape including three or more steps the pattern being continuous oversequential cycles, the first light distribution section outputs thelight, reflected by the optical disc and diffracted by the first area ofthe first light distribution section separately from the firstdiffraction light, as fifth diffraction light, and outputs the light,reflected by the optical disc and diffracted by the second area of thefirst light distribution section separately from the second diffractionlight, as sixth diffraction light, and a light amount of the firstdiffraction light and a light amount of the fifth diffraction light bothoutput by the first light distribution section are different from eachother, and a light amount of the second diffraction light and a lightamount of the sixth diffraction light both output by the first lightdistribution section are different from each other.
 16. An optical discapparatus according to claim 15, wherein the first diffraction light andthe second diffraction light output by the first light diffractionsection are positive first order diffraction light, and the fifthdiffraction light and the sixth diffraction light output by the firstlight distribution section are negative first order diffraction light.17. An optical disc apparatus according to claim 16, wherein a lightamount of the negative first order diffraction light is substantiallyzero.
 18. An optical disc apparatus according to claim 16, wherein alight amount output by the first light distribution section is largestfor the positive first order diffraction light, second largest for thetransmission light, and smallest for the negative first orderdiffraction light.
 19. An optical disc apparatus according to claim 16,wherein a light amount output by the first light distribution sectionlargest for the transmission light, second largest for the positivefirst order diffraction light, and smallest for the negative first orderdiffraction light.
 20. An optical disc apparatus according to claim 16,wherein a light amount output by the first light distribution section islargest for the transmission light, second largest for the negativefirst order diffraction light, and smallest for the positive first orderdiffraction light.
 21. An optical disc apparatus according to claim 1,further comprising a second diffraction light detection section,wherein: the first light distribution section includes a ninth area anda tenth area, the first light distribution section outputs the lightreflected by the optical disc and diffracted by the ninth area of thefirst light distribution section as third diffraction light outputs thelight reflected by the optical disc and diffracted by the tenth area ofthe first light distribution section as fourth diffraction light,outputs the light, reflected by the optical disc and diffracted by thefirst area of the first light distribution section separately from thefirst diffraction light, as fifth diffraction light, and outputs thelight, reflected by the optical disc and diffracted by the second areaof the first light distribution section separately from the seconddiffraction light, as sixth diffraction light, the second diffractionlight detection section includes an eleventh area, a twelfth area, athirteenth area, a fourteenth area, a f if tenth area, and a sixteentharea, a seventh sub-diffraction light detection section is provided inthe eleventh area, an eighth sub-diffraction light detection section isprovided in the twelfth area, a ninth sub-diffraction light detectionsection is provided in the thirteenth area, a tenth sub-diffractionlight detection section is provided in the fourteenth area, an eleventhsub-diffraction light detection section is provided in the fifteentharea, a twelfth sub-diffraction light detection section is provided inthe sixteenth area, the third diffraction light is detected by theseventh sub-diffraction light detection section and the eighthsub-diffraction light detection section, the fourth diffraction light isdetected by the eleventh sub-diffraction light detection section and thetwelfth sub-diffraction light detection section, the fifth diffractionlight is detected by the tenth sub-diffraction light detection sectionand the eleventh sub-diffraction light detection section, the sixthdiffraction light is detected by the eighth sub-diffraction lightdetection section and the ninth sub-diffraction light detection section,and the control device obtains a focusing error signal for the opticaldisc based on a difference between a total light amount of thediffraction light detected by the seventh sub-diffraction lightdetection section, the ninth sub-diffraction light detection section andthe eleventh sub-diffraction light detection section, and a total lightamount of the sub-diffraction light detected by the eighthsub-diffraction light detection section, the tenth sub-diffraction lightdetection section and the twelfth sub-diffraction light detectionsection.
 22. An optical disc apparatus according to claim 1, furthercomprising a second diffraction light detection section, wherein: thefirst light distribution section includes a ninth area and a tenth area,the first light distribution section outputs the light reflected by theoptical disc and diffracted by the ninth area of the first lightdistribution section as third diffraction light, outputs the lightreflected by the optical disc and diffracted by the tenth area of thefirst light distribution section as fourth diffraction light, outputsthe light, reflected by the optical disc and diffracted by the firstarea of the first light distribution section separately from the firstdiffraction light, as fifth diffraction light, and outputs the light,reflected by the optical disc and diffracted by the second area of thefirst light distribution section separately from the second diffractionlight, as sixth diffraction light, the second diffraction lightdetection section includes an eleventh area, a twelfth area a thirteentharea, a fourteenth area, a fifteenth area, and a sixteenth area, aseventh sub-diffraction light detection section is provided in theeleventh area, an eighth sub-diffraction light detection section isprovided in the twelfth area, a ninth sub-diffraction light detectionsection is provided in the thirteenth area, a tenth sub-diffractionlight detection section is provided in the fourteenth area, an eleventhtenth sub-diffraction light detection section is provided in thefifteenth area, a twelfth sub-diffraction light detection section isprovided in the sixteenth area, the third diffraction light is detectedby the seventh sub-diffraction light detection section and the eighthsub-diffraction light detection section, the fourth diffraction light isdetected by the eighth sub-diffraction light detection section and theninth sub-diffraction light detection section, the fifth diffractionlight is detected by the tenth sub-diffraction light detection sectionand the eleventh sub-diffraction light detection section, the sixthdiffraction light is detected by the eleventh sub-diffraction lightdetection section and the twelfth sub-diffraction light detectionsection, and the control device obtains a focusing error signal for theoptical disc based on a difference between a total light amount of thediffraction light detected by the seventh sub-diffraction lightdetection section, the ninth sub-diffraction light detection section andthe eleventh sub-diffraction light detection section, and a total lightamount of the sub-diffraction light detected by the eighthsub-diffraction light detection section, the tenth sub-diffraction lightdetection sections and the twelfth sub-diffraction light detectionsection.